June 10, 2019 1

A recent fire at a utility-owned energy storage facility near Phoenix, Arizona has implications for everyone who is standardizing around lithium-ion batteries to design storage systems. Since lithium represents about 95 percent of the market, this is a topic of near-universal interest.

Especially for me. My experience with lithium-ion batteries goes beyond the storage system engineering and design work we do here at SepiSolar. Ten years ago, not long after founding SepiSolar, I helped launch Green Charge Networks, an early leader in lithium battery deployments. That’s where I saw technology, product configuration, permitting, performance, and operational risks associated with lithium batteries begin to materialize.

The industry is moving fast to push lithium battery deployment to new heights, but we still cannot easily quantify risks. Nor the costs.

We at SepiSolar are technology agnostic. Our commitment is to openly consider the costs and benefits of all commercially viable design options. Given how many projects appear to be treating lithium as the only commercially viable technology, I encourage developers to reevaluate lithium—particularly after the recent fire in Arizona—and consider flow batteries as an alternative that can be deployed at lower cost, greater speed, and superior safety.

Arizona storage system fire

Reported facts about the fire at the McMicken Energy Storage facility are limited. Based on local media reports, we know that firefighters responded to an incident on April 19. While inspecting the 2 MW / 2 MWh battery system, eight firefighters suffered injuries in an explosion. The cause is unknown. All of the firefighters with the most serious injuries were in stable condition in the days following the blast.

The system owner, Arizona Public Service, switched off other energy storage projects in the aftermath of the fire but, as Greentech Media has reported, APS is not wavering on plans to deploy 850 MW of battery storage by 2025.

The entire industry will be paying close attention when investigators reveal their findings about the root cause of the fire and the ensuing sequence of events. Even now, however, developers can size up the inherent risks that all projects using lithium-ion batteries should address.

Lithium battery risks

Eight years ago, when the US Department of Energy awarded Green Charge Networks a $12 million grant to deploy lithium battery storage systems, I was bullish on the technology. If anyone was a believer in lithium, it was me. Then, one by one, the following risks came to light.


Every time you cycle a battery, capacity and efficiency drops bit by bit. Performance on day 30 will not be the same as on day 1. How well does your financial model sold to your client calculate degradation along the performance line?

Thermal runaway

It’s critical to make sure that a battery operates according to its specification. This means when you integrate lithium batteries at a facility, the function of the HVAC system expands from comfort to safety. Now, when an HVAC system requires a little maintenance, it’s not just an O&M concern, but a safety risk. A battery that begins to operate outside of its normal temperature range can experience thermal runaway.

Hazardous materials

Lithium-ion batteries use materials that can introduce safety and environmental hazards if not properly contained. The storage industry needs an effective process for salvaging lithium, nickel, cobalt, or manganese. There’s no need to reinvent the wheel. We can borrow best practices from the solar industry, which has recycling for silicon-based solar modules and collection and recycling of cadmium-telluride thin film modules at the end of their operating lifetime.

Warranty claims

Lithium battery vendors have not yet established a track record showing the warranty claims rate. Or the frequency of warranty claims, which reflects the long-term failure rate for systems operating in the field. Early adopters carry the risk that failures may exceed expectations, straining the supplier’s ability to make good on all claims. In fact, engineers at one large lithium battery supplier have published a peer-reviewed scientific paper saying that lithium batteries are degrading faster than expected and proposing a patent to resolve the issue, suggesting that the risks should be taken seriously.

Human rights

Three years ago, the Washington Post published an expose on cobalt mining practices in Congo, where children help populate the workforce that uses hand tools to dig in underground mines, exposing themselves to health and safety hazards. Cobalt is an ingredient in lithium batteries. The Post has also traced the lithium supply chain from parts of Chile, where indigenous communities have struggled to protect the environment and win local economic benefits from the extraction and sale of the lucrative mineral.

Downstream costs

The low upfront cost of lithium batteries is only part of the total cost of ownership, one that excludes downstream costs associated with operations and maintenance of the batteries, the fire detection / suppression system, or the HVAC system that keeps the batteries within their specified temperature range. A failure to perform proactive operations and maintenance could not only increase long-term costs but void the manufacturer’s warranty.

Parasitic load

As industry analysts have gained a deeper understanding of how much storage capacity is needed to keep storage-integrated HVAC systems running, it appears that round-trip efficiency for lithium battery systems may be lower than originally thought. Citing Lazard’s ongoing levelized cost of storage analysis, Greentech Media has reported that parasitic loads could knock down system efficiency by 17 percent or more.

Advantages of flow batteries

In recent years, I have had many opportunities to compare lithium batteries and vanadium flow batteries side by side, while designing storage systems at SepiSolar and performing battery tests in partnership with Nextracker. The battery test, ongoing since 2017, consists of over two dozen battery types, including 5 lithium batteries, 6 flow batteries and 2 flywheels, plus an ultracapacitor, an advanced lead-acid battery, a copper-zinc battery, and a nickel-iron battery.

Through firsthand experience, one key observation at this point is that the market currently has two leaders in the race to achieve lowest total cost of ownership: lithium batteries and vanadium flow batteries. Vanadium flow batteries have earned a place on the leaderboard based on advantages in cost, performance, installation speed, safety, and design simplicity.


Please note, first of all, that battery costs vary based on storage system design and use case. The battery cost for a commercial system used principally for demand-charge reduction will be different than the battery cost for a grid-scale storage project designed for transmission and distribution deferral.

That said, Nextracker has shown that vanadium flow batteries can yield a lower total cost of ownership than lithium batteries due to significantly lower O&M costs over 20 years.


Nextracker has also demonstrated a competitive installation process with vanadium flow batteries. Installation of Nextracker’s NX Flow, a solar-plus-storage solution using Avalon Battery’s vanadium flow battery, requires less installation time and fewer materials than a central storage system due to being shipped “wet.” This means it’s full of electrolyte from the factory. It’s the first battery in the world to demonstrate this feature.

The battery is pre-commissioned and integrated with a 3-port string inverter at the factory. All battery-to-inverter wiring is complete on arrival. Before installation, the construction crew drives piles and installs cross rails to set up a mounting platform. Then the crew places the battery with a forklift and bolts the battery to the platform. Finally, the crew connects DC and AC wiring from the solar array to the inverter. Here is a 3-minute demonstration.


All plated batteries, including lithium batteries, have inherent safety risks. If you take the positive and negative sides and create a short circuit, the wire can get so hot that it explodes. Firefighters have reported on fires in electric vehicles that get damaged in a car crash, get towed, and catch fire days later.

Vanadium flow batteries have three key safety advantages. First, you can turn a vanadium flow battery off, preventing the device from charging or discharging altogether, and with zero voltage on both the positive and negative terminals. Second, the temperature rise in a flow battery is limited. Even if you short the battery on the chemical side or the fluid side, the temperature rises briefly and then drops, and the battery can be placed back into service immediately with no downtime to speak of. It’s the most boring test you’ll ever see. Finally, there are no flammable, toxic, nor hazardous materials or components. Check out this white paper on energy storage system safety from retired San Jose Fire Captain Matthew Paiss to learn more.

Battery design

Flow batteries are simple by design. They consist of two chemical solutions, one with positively charged ions, another with negatively charged ions. When connected to a generator (actually, a reversible fuel cell) the battery charges by pulling ions from the positive solution and pushing them into the negative solution. When you switch the battery to discharge, the ion flow goes in reverse and generates an electric current. The “secret sauce” of vanadium flow batteries is that the entire electro-chemical reaction happens in a purely aqueous state, which translates to “no degradation,” which translates to “lowest LCOS.”

Trust and visionary thinking

As a licensed engineering firm, SepiSolar’s first obligation is to follow national and jurisdictional codes and standards. The value of our design work depends on our ability to optimize the best products and technologies for the right applications that maximize benefits and minimize costs, all while providing structural and electrical engineering stamps in all 50 states. Beyond that, SepiSolar follows a set of core values that promotes trust and integrity, and encourages visionary thinking.

When customers approach us to design lithium batteries for residential and commercial applications, we do it. When customers ask us to advise them on the tradeoffs between battery technologies, we do that as well, covering all the topics raised here.

Our commitment to promoting trust and visionary thinking compels us to discuss openly the risks (and, therefore, costs) of lithium batteries, especially in the aftermath of the Arizona storage system fire. While we hope the industry can mitigate all the risks, many have not yet been fully addressed. Meanwhile, we owe it to the Arizona firefighters who suffered injury to engage in an open discussion about lithium batteries.

Our customers are remarkably entrepreneurial. We expect that contractors will quickly adapt to market changes by delivering storage solutions that balance cost, speed, and safety.

Please contact us if you want to learn more about engineering and design for storage systems using flow batteries.

Josh Weiner


February 1, 2019

When I saw this article about LG lithium-ion energy storage fires in Korea, I couldn’t help but think of the fires that PG&E is being held responsible for in California. Those fires have ultimately lead PG&E into bankruptcy and will inevitably increase energy costs to ratepayers.

It’s amazing how something as seemingly simple as a campfire, power line, or a 18650 lithium cell—about the size of a lipstick container–can cause so much damage to California, one of the wealthiest states in the world and PG&E, the largest utility in the state, and, of course to the loss of lives and homes.

Some of these hazards defy logic or at least expectations. When SepiSolar was providing technical due diligence and engineering review services to NRG Home Solar from 2014 – 2016, we came across residential projects on the East coast that had unexpected dangers. For example, there was a solar PV system installed on top of the garage where snow had piled up on the PV system. Some rain had turned that snow into a giant slab of hardened ice. When the ice slipped off the solar array, it crushed the car parked in the driveway–not dented, dinged, or scratched. It completely totaled the car. The homeowner told us “that’s exactly where my children play in the summertime.”

Having just become a father at the end of December 2018, I think it’s fair to say that safety cannot, should not, and will not ever be taken for granted on my watch.

Risks vs Benefits

I don’t mean to suggest that we ought to over-design, over-engineer, over-regulate, over-install, or somehow bullet-proof every single component or assembly in a traditional solar or storage system.  That’s like saying “Since car accidents kill people, let’s require everyone to drive army-grade tanks down the street.” That line of thinking effectively kills an industry and becomes a zero-sum game. Instead, I would pose that taking risks is a part of life and is healthy for us, since taking risks and stepping outside our comfort zones is exactly how we grow, learn, and evolve.

The goal is to take calculated risks, or, alternatively, educated risks. What’s a calculated risk? It’s a risk that you’re aware you’re taking. The difference between educated risks and blind or reckless risks is awareness.

We then need to weigh those risks against the benefits in order to make effective decisions. After those decisions are made, we need to be ready to revisit them again soon because the learning process never stops. Assumptions will need to be revised, data recalculated, risks revisited, benefits re-weighed, and decisions re-evaluated. This is how we evolve and approach an ever-safer future, together.

So, let’s build some awareness, shall we? Let’s have a data-driven discussion about the fire risks associated with energy storage systems, and let’s turn our blind risks into calculated ones. Having helped build Green Charge Networks into a nationwide energy storage integrator (acquired by Engie in 2015), engineered solar and battery systems for over 10+ years, and having worked with utilities, UL, code officials, etc. on safety standards, I think I might have a thing or two to say about this subject.

Evaluate the Energy Storage Technology

To minimize risks in energy storage, perhaps the most obvious approach is to work with a technology that inherently works with chemicals and materials that have no fire risk associated with them. This is particularly difficult with batteries because when almost any battery is short-circuited, they instantly become a fire hazard. But that’s the nature of batteries – they can produce insanely high amounts of current, since the resistance in the battery circuit is governed by however fast (or slow) the chemicals involved can react with each other, allowing the free flow of electrons to accumulate. Of course, these chemicals are designed to react with each other in order to release electric charge. So, fire hazard is almost inherent in any battery (with at least 1 exception).

I love this side-by-side technology comparison authored by Fire Captain Matthew Paiss, a 22-year veteran of the San Jose Fire Department. Captain Paiss is the Fire Department’s subject matter expert on energy storage and is the IAFF primary representative to NFPA 70 (National Electrical Code) and NFPA 855 (Energy Storage System Standards), which has been incorporated into UL standards such as UL 9540. It was surprising and gratifying to know that there’s at least 1 technology that rises above the rest when it comes to safety.

Codes & Standards

There are a ton of uber-smart tradesmen, engineers, officials, and subject matter experts who love to wordsmith and craft codes and technical language (God love them!) in order to impose a minimal, universal set of health and safety standards designed to protect personal property and life. Some of these codes go all the way back to 1897, as is the case with the National Electrical Code, when electricity was thought of as a liquid! (Check out Leyden jars.)

Bottom line, let’s be sure to read and understand the modern codes thoroughly, including NFPA, NEC, UL, among others. Every word, comma, and comment were crafted with the care one would expect of a nationally applicable set of requirements, even if you disagree with many of them. It’s important to follow voltage, current, and sizing requirements, naturally. NEC 706, for instance, was just added to the NEC in the 2017 edition. That’s the first time batteries have been overhauled in the NEC since Article 480 was written back in the early 20th century! Let’s expect this new code section to evolve with the times as more data becomes available and continue to think of these codes as a “minimal” set of safety standards that we can go above-and-beyond as necessary to ensure the safety of the systems we design and build.

Real-time Data

While codes and standards are important, one of their drawbacks is that they are slow to change. Technology and data often evolve faster than codes and policies. Because of this, it’s important to look at the data, stay up-to-date on the latest-and-greatest information available, and dynamically build this data into your systems as it becomes available. Basically, I’m advising you to read. Read articles, publications, journals, media newsletters, and absorb as much as possible to keep up-to-date.

For instance, now that the above Korean article has surfaced about LG battery fires, it’s imperative to find out the root cause failures that led to these hazards. There is much to learn from failure, thereby converting failure into learning opportunities (which perhaps negates the use of the term “failure” in the first place – nothing is a failure, so long as you learn something from it!). We don’t have to wait for new technologies or new codes to come out. Instead, let’s use the data right away in any or all systems that we may be using with LG batteries, or any battery, for that matter.

The first time I thought about the risks associated with batteries was when I heard that Boeing grounded the Dreamliner. Our Co-Founder and CEO of Green Charge Networks at the time was a retired Boeing executive, so this naturally caught our attention. Wikipedia does a decent job summing up that experience, and you can get the full investigative report here.

The general takeaway is that regulatory bodies, manufacturers, and engineers were not “up to snuff” on the risks associated with battery technology. To a great degree, as the above Korean article shows, we are still learning these risks. At our time at Green Charge Networks, we understood that this meant that the safe deployment of battery systems would largely rest on us, since codes, standards, products, and regulations were still too much in their infancy to support us.

Direct Experience and Training

Nothing prepares you for danger, uncertainty, or risk more than education, experience, and training. The more hands-on experience you have with a particular product or technology, the more you will understand its limitations, weaknesses, and risks. Understanding not only what and when a battery undergoes thermal runaway, but also the “how” can really help put battery risks into perspective. What I learn from this is that it’s not just the battery one should be cautious of, but also the environment the battery is in. For example, does the battery have a fire suppression system? Is the battery located near any buildings or structures that have no fire suppression?.

One time I dropped a wrench on an old golf cart battery, and it just so happened that the wrench landed perfectly on both positive and negative terminals simultaneously. It was the first time I saw metal turn bright red, orange, and then white, and eventually melting all over the battery. This was just a regular ol’ lead acid battery, so it was surprising to me that such an old battery could have such a great impact on something as solid and stiff as a wrench. Needless to say, I am very cautious around terminals of batteries, since most batteries cannot be inherently turned “off” (again, with some exceptions).

In a nutshell, if you’re working with lithium batteries, make sure to identify the risks and retire them as much as possible. For instance:

  • HVAC systems for lithium are not just there to support battery performance, but they are safety devices as well. Make sure they’re appropriately sized and adequate for the operating environment the batteries will be in.
  • Lithium batteries that get too hot can result in thermal runaway, and other types of hazards, aside from accelerated degradation of the cell capacities and efficiencies. Fire suppression systems are required with the appropriate cleaning agents.
  • Closely monitoring and isolating cells that are approaching their end-of-life is critical. Battery degradation not only leads to capacity loss, but also battery failure.

There are many other aspects to keep in mind, and nearly all are avoidable if you’re aware of them in the first place.

I strongly believe that lithium-ion battery systems will continue to grow and thrive in our new renewable energy world, but as the Korean article shows, there are risks. As engineers, it’s our responsibility to be aware of these risks, evaluate them, and to find the solutions that will decrease those risk and perhaps even eliminate them with new safety innovations.

Josh Weiner


January 24, 2019 1

UPDATED: The CPUC has unanimously passed this California NEM Storage decision on January 31, 2019. The information in our white paper reflects the final decision.  

In December, SepiSolar published a white paper that reviews a proposed CPUC decision to include net energy metering (NEM) with DC-coupled energy storage for commercial solar systems. As of the writing of this blog post, the CPUC is set to vote to finalize the decision on January 31, 2019, and is expected to pass. However, it’s possible the vote will be postponed due to other priorities, such as PG&E’s bankruptcy filing. (Check the latest CPUC agenda here.)

While our white paper describes many of the financial benefits to the decision, several energy storage and inverter manufacturers had questions about the firmware solution that we designed for NEXTracker’s NX Flow system, a DC-coupled energy storage system.

Below is a list of some of these questions and the answers. As always, if you have more questions, please submit them in the comments section or send them to [email protected]

Is DC-coupled storage with net metering approved in California only with NEXTracker’s NX Flow product?

As soon as the CPUC approves the policy change (hopefully by the end of January 2019), the NX Flow would be immediately eligible, since its firmware has already been verified by UL. However, other DC-coupled storage manufacturers may design similar firmware for their products. Eventually, UL will update their 1741 standard to include these protocols. In the meantime, utilities are allowing discretionary approvals of this policy, even though the CPUC hasn’t fully adopted it yet.

The white paper says that SepiSolar co-developed the firmware. Does that mean that energy storage or inverter OEMs need to license the code from SepiSolar or NEXTracker?

No. SepiSolar wrote the specifications, designed the testing protocol, and demonstrated the underwriting and verification process with our client, NEXTracker.

As with NEXTracker, OEMs will need to develop their own code and implement into their California NEM/Rule 21 compliant product after UL verification. Based on our experience, a manufacturer can typically develop the code within a day or so.

While SepiSolar does not write the firmware code, as an independent engineering firm, we’re able to help inverter and energy storage manufacturers with the functional and technical requirements to comply with this updated NEM energy storage policy for DC-coupled systems. Having gone through the UL process ourselves, we can advise on firmware design, testing pain points, pitfalls, and how to get through the UL approval process as expeditiously as possible.

Eventually, UL will update its 1741 standard to include the protocols that SepiSolar developed.

What do you mean by “firmware”? Don’t you mean “software”?

In order to adjust to this NEM storage proposal, utilities asked that the associated OEM software not be changed after interconnection, and that it be “hard-coded” into the hardware device’s “firmware” itself. They wanted to be certain that nobody could come back to the system later, after PTO (Permission to Operate) was issued, and re-program the battery to charge off the grid, thereby breaching the system’s interconnection agreement with the utility company.

While “firmware” involves software coding, it’s typically installed once at the manufacturer’s facility and implies that the software can’t be modified after installation or interconnection. On the other hand, “software” is inherently adjustable and can often be updated remotely by the system owner, OEM, or even third parties.

As a result, the “NEM software” (firmware) cited in the proposed CPUC decision must be hard-coded into the DC-coupled inverter device. It must then be recorded, tested, and verified by a Nationally Recognized Testing Laboratory (NRTL), such as UL or TUV. The inverter product must also have a specific version number and checksum that cannot be confused with other non-NEM-compliant hardware.

In the future, it’s possible that we’ll find ways to allow “un-editable” software to be located outside of the DC-coupled inverter device, perhaps in an EMS (Energy Management System) controller. However, the EMS would need to prove to the utilities that it, indeed, cannot be updated post-PTO. These alternatives are currently being discussed.

What happens if you update the firmware after interconnection?

As mentioned, the firmware protocol that SepiSolar developed ensures that the software is hard-coded by the OEM and verified that it was installed correctly by an NRTL. In our UL-verified protocol, if the firmware is changed after installation or interconnection, it will necessarily void the UL verification and put the entire installation in breach of its interconnection agreement (and Rule 21) with the utility. The utility will then be able to shut down the system and potentially fine the customer for any damages the utility may have sustained for the breach.

What are the firmware requirements?

The firmware requirements that any DC-coupled system would need to satisfy to receive NEM credits are fairly straight forward. It must be designed so that the battery can never charge from the grid. In addition, the firmware solution must be tested and verified by an NRTL, such as UL.

If you’d like to learn more about the specific tests themselves (there are 5 total), feel free to reach out to us at [email protected], and we can share, specifically, what these tests entail. In summary, the tests involve:

1) The inverter’s ability to sense a potential “battery-charge-from-the-grid” event (which would violate this NEM policy) and mitigate it by controlling a DC bus voltage in order to turn the battery “off,”
2) The battery’s ability to be turned “on” or “off” by the inverter vis-à-vis this DC bus voltage control method described in (1) above,
3) The verification of software version number,
4) That no other software-controlled device (like an EMS) can override the inverter’s firmware, and
5) Sensitivity testing on all the above in the event that the PV supply varies widely (say, with variable cloud cover events).

Instead of inverters, can DC-DC converters adopt the firmware?

Yes. We see a clear use-case for getting a DC-DC converter approved under California NEM, but it would require a slightly different testing regime than the one we’ve developed for NEXTracker’s NX Flow product.

Is NEM with DC-coupled storage only available in California?

Yes, for now, due to this pending CPUC policy change, DC-coupled NEM with energy storage will only be available in California. However, other states typically follow California’s lead with policies like this. As of mid-January, 2019, we haven’t seen a system get approved outside of California.


We hope these responses answer your questions about the UL-verified firmware that is required for DC-coupled energy storage. If you have further questions, please add a comment or contact us at [email protected]

Josh Weiner


December 8, 2018 0

As we approach this holiday season, I’d like to take a moment to look back at 2018 and share some of what’s in store at SepiSolar for 2019.

First, on behalf of our entire team, we want to thank you for selecting SepiSolar as your 2018 solar design and engineering partner. This year has seen many changes in the solar industry–as well as at SepiSolar.

In addition to our usual design work, we redesigned our logo and then redesigned a new HQ, expanding to larger offices in Fremont. The move was largely due to adding new team members to our engineering and operations teams, enabling us to design more efficiently and deliver plans on time.

Along with new team members, SepiSolar instituted new quality control measures and new design tools that are helping SepiSolar engineers design solar-plus-storage systems with increased speed and accuracy. In fact, we’re proud to report that nearly 90% of our customers’ residential designs receive permits without any revisions. For commercial, industrial, agricultural, and multi-family projects, 80% of projects receive a permit without revisions, even in America’s most demanding jurisdictions.

2018 also saw the launch of several new services, including Salesforce consulting for solar companies and Sepi Academy, our new NABCEP solar and energy storage training program.

What’s in Store for 2019

For 2019, SepiSolar will be keeping up with all the new permitting changes here in California and across the U.S. With a 100% renewable energy goals set for Hawaii and California, plus California’s new Title 24 solar roof mandate, we’ll be informing our clients on all the latest requirements and best design practices.

You’ll also see a new SepiSolar website that will be easier to use and filled with more resources, such as our site survey checklists and more new downloads and White Papers. Of course, we’ll also be generating new useful blog content and continuing our Ask SepiSolar Anything webinar series.

Thank you for being a part of SepiSolar in 2018. We’re excited for 2019 and look forward to working together on bringing more GW of solar and energy storage to the U.S. and the world.

From all of us at SepiSolar, we wish you and your family a wonderful holiday and a prosperous and happy new year!


Josh Weiner, CEO of SepiSolar

Josh Weiner


May 30, 2018 0


If you’re an architect, new homebuilder or housing developer in California, you’ve probably heard by now that the California Energy Commission (CEC) has updated its Title 24 solar and energy efficiency standards. Effective January 1, 2020, the update specifically mandates that all new California homes under three stories install solar panels on the roof or achieve an equivalent total home energy efficiency reduction through other measures.

To comply with Title 24, new homebuilders, architects and developers will be required to use Title 24’ssoftware for calculating the building’s “Energy Design Rating” (EDR), which not only includes inputs for solar but also for energy storage and other options.

To give builders more flexibility, the EDR is scored like a golf tournament—the lower the score, the better (or, the more “energy efficient” the home is). The goal is to achieve an equal-to-or-less-than EDR for a solar home than a comparable “regular” home, of the same square footage.

Depending on the square footage and climate, new-home solar will range between 2.7 and 5.7 kW DC to meet the requirements, but that doesn’t tell the whole story. Rather than meeting the minimum requirements, builders may be better off designing their Title 24 solar systems with battery storage.


Why Builders Should Include Storage with Solar

As mentioned, the EDR software gives homebuilders a score, but there are many ways to meet that score, and one is combining solar with energy storage. Including energy storage will not only meet the minimum solar requirements, but will maximize energy savings for the home, offering customers a financial advantage over other homes.

When solar engineers design a solar system, they typically take into consideration the following factors:

  • The climate
  • The average amount of sunlight for the area
  • The orientation of the roof in relation to the sun
  • The amount of potential shading over the course of the year
  • The pitch of the roof
  • The home’s annual kWh usage

While these parameters are important, equally important are the utility rate considerations that system designers like SepiSolar factor into their plan sets. These rate policies affect the solar system’s ROI and include:

  •  Tiered Rates. Tiered rates vary by utility and charge customers higher rates when they use more energy over a certain monthly amount.
  • Net Energy Metering (NEM). NEM is like rollover minutes for solar. Utilities will credit solar homeowners for any excess solar power that is exported to the grid. The value of NEM varies by the utility and the time of day that the solar is exported to the grid.
  • Time of Use (TOU). TOU rates also vary by utility. Customers incur charges when they use grid energy. During peak times, such as rush hour when the sun is setting and people are coming home, utilities charge solar and nonsolar homeowners a higher rate when they draw power from the grid, making any exported solar energy less valuable during that time of day.

That’s where energy storage (batteries) comes in.


Designing Systems for Overall Cost Savings for Solar and Title 24

Due to the above utility rate considerations, home developers that want to design premium homes that maximize utility savings as well as comply with the Title 24 solar mandate should consider including energy storage systems with their solar designs.

Solar+storage with smart battery management software will counteract the cost of tiered rates and TOU through “load shifting,” and “peak shaving.”

With peak shaving, homes using solar+storage will be able to use as much free solar as they can during the highest TOU rates while saving the excess energy in their batteries instead of exporting to the grid. Then, during peak TOU periods, the home will use this free stored solar-generated energy when the utility rates are high.

Additionally, battery management systems can also “load shift” the time when appliances are turned on or off, such as turning on a dishwasher, dryer or charging an EV when utility rates are low or when electricity can be drawn from the battery that was charged by solar.

Both peak shaving and load shifting with solar+storage encourage the home to use more of its own self-generated power, relying less on importing power from the grid. With so many homes using solar after 2020, homeowners with solar+storage will also help stabilize the grid, and can be paid a higher credit for any power the utility draws from the storage system during peak hours.

Another sales advantage for developers is that solar+storage offers some emergency power in case of a blackout. That is not the case with stand-alone solar PV systems. To protect power line repair workers, stand-alone solar systems will automatically shut down during an outage.


Things to Keep in Mind About Solar+Storage with Title 24

If you decide to meet your Title 24 solar mandate with energy storage, there are several requirements to keep in mind.

First, when adding storage to solar, there is a minimum required battery size of 5 kWh. This is a reasonable size that will allow for taking advantage of tiered rates and TOU, and it will provide a minimal amount of backup power in case of an outage.

Second, your solar+storage system designer and engineer will have to select one of three control options for the battery:

  • Option 1 – Basic Control (Title 24, Section JA12.2.3.1): With Basic Control, the battery system can only be charged by the solar system and can only discharge when there’s not enough solar power to meet the home’s current energy usage.
  • Option 2 – TOU Control (Title 24, Section JA12.2.3.2): With TOU Control, battery system will be set up with Basic Control, but will only discharge during the peak TOU hours of the day. This will change from season to season, and must be configured from the battery manufacturer or programmed by the installer at the time of commissioning.
  • Option 3 – Advanced Demand Response Control (Title 24, Section JA12.2.3.3): With this configuration, solar+storage systems will be programmed with Option One or Two. In addition, the battery control system must meet the demand-response requirement of a utility or third-party owner; that is, the utility or third party will be able to remotely control when the battery is charged and discharged. Typically, the homeowner will receive a financial benefit for this utility interaction with the grid.

The rules within each category will most likely be refined over time, so it’s important for your solar designer to be up to date with these standards and make any necessary changes. The above is a summary, so please review the entire Joint Appendix 12 to take full advantage of the above credits.

As longtime solar+storage engineers with thousands of projects, SepiSolar has a great deal of experience designing solar and battery systems that meet the new Title 24 regulations, as well as designing systems that comply with the local requirements of counties and other local authorities having jurisdiction (AHJs). Please contact us if you have any questions about these new Title 24 solar requirements for your new residential solar development projects.

Josh Weiner is President and CEO of SepiSolar, a solar+storage design & engineering firm based in Fremont, CA.

Josh Weiner


May 4, 2018 1

Have you seen SepiSolar’s new logo? It used to look like this:

And now it looks like this:



When any company goes through a redesign of their brand, customers and frequent visitors can notice and often have an initial reaction (they love it, they hate it, meh.) as well as questions, like “What was wrong with your old logo?” and “What does your new logo mean?” and “What does the number 42 and meaning of life have to do with solar design and engineering?

These are all great questions, so let’s tackle them one by one:

What was wrong with your old logo?

Sometimes our solar engineers can be checking a permit plan set and spot something on the plans that doesn’t match or make sense. That’s how we felt about our old logo. The solar panel array made us look like a solar developer or EPC, and while solar contractors are certainly our customers, a solar array is not a reflection of design or engineering or who we are beyond being in the solar business.

Second, when you look at a brand, it’s supposed to reflect a feeling. Think about Nike and that swoosh of energy. We want the solar industry to recognize SepiSolar in that same way, to see our logo and have a positive feeling, and our old logo never did that. It was “just a logo.”

What does your new SepiSolar logo mean?

Designing and engineering a logo is serious business, and we did consult with a professional who asked us a lot of questions about who we are and what does our SepiSolar name mean?. The short answer is that “Sepi” means “the moment just before the first light of dawn.” So, light and energy emerge and grow from Sepi, and in the same way, solar projects and the power that is later generated emerges out of the plan sets that SepiSolar’s engineers create. Power also emerges out of the engineering solutions we solve as solar consultants.

Our logo designer created several options to have our new logo reflect who we are as a company, but this was the one that immediately resonated with us. Here’s why: 


“42 and The Meaning of Life”

As to why we put 42 in the title of this post, if you didn’t catch it immediately, the number 42 is an important but ridiculous part of Douglas Adams’ book “Hitchhiker’s Guide to the Galaxy.”

It’s relevant to our SepiSolar brand because we sometimes feel like we’re part of that ridiculous universe when we deal with the bureaucracy of solar and battery permitting requirements, but it’s not part of our logo. We just thought it would be fun to slip 42 into a blog post title, and fun is part of our community of solar “enginerds,” so 42 is relevant…sort of.

Thanks for reading, and if you’ve missed our latest SepiSolar news and useful info like our SepiSolar Battery Translator Tool, please sign up for our newsletter or check out other news and blog posts! Naturally, please contact us if you’d like a free quote for our engineering services.

Josh Weiner is President and CEO of SepiSolar

Josh Weiner


April 10, 2018 2

“What does the SepiSolar name mean?”

“Josh, how did SepiSolar get started?”

“What makes SepiSolar different from other solar design and engineering services?”

Since founding SepiSolar nearly 10 years ago, those have been the most frequent questions I’ve been asked when people hear about SepiSolar. If you’re learning about us for the first time, or you’ve been a customer and wondered about those same topics, here’s the story:

“What Does the SepiSolar name mean?”

The short answer is that while searching for a name, I discovered that “Sepideh” is a Persian name that is often shortened to “Sepi.” Roughly translated, Sepi means “the moment just before the first light of dawn.” In other words, every morning, anywhere in the world, light and energy emerge and grow from Sepi.

In the same way, solar projects begin with solar design, and all of the clean power that is later generated emerges out of the plan sets that our designers and engineers create.

The Sepi “emerging light and power” concept also suggests infinite clean power generation. Even through the darkest solar coaster times, the sun always rises, and more plans for solar and storage are continually being generated for our customers. Moreover, our engineers are continually thinking about how to improve solar and storage technology, policies, business processes and grid models. The clean energy innovation never stops.

“How did SepiSolar get started?”

I founded SepiSolar in 2008, but my solar career started in 2004 with Andalay Solar (fka Akeena Solar). As one of the first national solar installation companies in the U.S., I learned a lot about the internal operations of a large engineering department, and realized there was a huge need for third-party independent design and engineering services to help growing solar companies during certain seasons, as well as for solar product development.

My vision was that SepiSolar would not only provide extra help to solar contractors with a high volume of designs and plan sets, but that we could also fill in the gaps for structural or PV electrical engineering needs that may be outside of the company’s core competencies, such as architecture firms or commercial building contractors.

I also wanted SepiSolar to be extremely flexible so that we could deliver a full menu of on-demand, seasonal or ongoing solar design and engineering services, such as installation feasibility evaluations, sales-focused drawings for proposals, P.E. stamps, product evaluations, or even develop salesforce modules for tracking inventory and the paperwork for projects.

With that flexible full-service mindset, SepiSolar has grown to become a leading national design and engineering company with a team of NABCEP certified designers and engineers working out of our Fremont, California offices, not overseas.

“What makes SepiSolar different from other engineering or design firms?”

I think of SepiSolar as a community of passionate “solar enginerds.” Everyone here looks at design and engineering through the lens of the entire solar and storage value chain. We don’t just draft line diagrams and crank out plan sets. For some design firms, that’s where the service starts and ends, but for SepiSolar, our services include our community of knowledge about the latest solar and storage technologies, policies, manufacturer relationships, and our experience with AHJs around the U.S. and abroad.

As a community of engineers, we’re also great communicators with each other and our clients. We regularly share information and complement each engineer’s knowledge base. And while some firms may chain their engineers to CAD monitors with MC4 connection cables, our SepiSolar engineers can also act as independent engineering consultants, visiting solar project sites, ensuring quality, improving O&M or troubleshooting commissioning. We also consult with manufacturers, developers, asset managers and storage companies, providing the entire SepiSolar team with a comprehensive and continuous feedback loop of information from all over the solar industry.

In addition to formal consulting, our designers also informally consult with clients at the start of every project. During these calls, we take into consideration the company’s preferences and various skill sets. For example, a roofer who installs solar may feel comfortable drilling holes into a commercial rooftop but prefer microinverters for simpler electrical work. Similarly, an electrician may be very comfortable with optimized string inverters, but prefer a ballasted roof design to avoid roofing issues.


Naturally, there’s much more to tell about how SepiSolar grew over the last 10 years and why we’re so passionate about everything we do. Perhaps the best way to learn more is to set up a free consultation with me or just get a quote for your next solar or storage project. You can also join our SepiSolar community by simply following us on LinkedIn, Twitter or Facebook, or joining our mailing list. Please reach out for any questions or comments.

Josh Weiner is President and CEO of SepiSolar.

Josh Weiner


March 27, 2018 2


Typically, the first figure developers and asset managers use to compare energy storage technologies is its dollar per kilowatt-hour ($/kWh) cost. But that seemingly simple metric may not be an accurate apples-to-apples one unless you’ve asked your battery manufacturer some important engineering questions.

To fairly compare the $/kWh cost of selected energy storage systems, we recommend your energy storage system designers ask the following four questions. In Part 2 of our series, we’ll explore technology considerations, applications and bankability.

Question 1: Is the battery vendor defining $/kWh in DC or AC watt-hours?

Storage, like solar, is becoming commoditized, so we need a common metric for pricing energy storage systems. Just like solar system dollars per watt pricing, you’ll need to have all vendors offer you pricing in either AC or DC kilowatt-hours. Their answers will lead to more questions; however, how are AC or DC kWhs defined? Which one includes the percentage of depth of discharge (DoD)? Are any parasitic loads included? What about DC-coupled systems, which may only have one inverter for both the PV and storage systems? Your engineer should know the answers to these questions to make a fair comparison of batteries.

Question 2: What about OpEx?

With batteries, it’s difficult to talk about capital expenditure (CapEx) without also talking about operating expenditure (OpEx). For instance, batteries are more expensive to maintain than PV systems due to their sensitivity to things like:

  • Temperature (battery HVAC systems require routine maintenance)
  • Fire suppression systems
  • Augmentation schedule (e.g., “capacity maintenance agreement” to replace degraded and failed battery modules over time as they wear out)

The last category mentioned above is particularly problematic due to the replacement schedule of batteries being closely tied to their use-case application. Some projects may require battery replacements within 5 years, while others in 12 years, and some last well beyond 20 years. Therefore, it’s important to take into account different duty cycles, technology choice, and specific use cases to determine the appropriate OpEx.

In addition, recent data has become available showing that actual battery performance is often different than what’s warrantied, which unfortunately translates into shorter-than-expected battery life spans, and, therefore, more cost to the project owner.

Moreover, considerations such as auxiliary loads, thermal management and round-trip efficiency (RTE) also degrade over time, resulting in higher battery system losses, and, therefore, less return on investment. These are all OpEx costs that can be carefully calculated and accounted for, but only when a thorough understanding of project economics and underlying product configuration and technology are combined to identify the true costs of ownership (more about this in question 3).

Question 3: Does your $/kWh figure include DoD and efficiency factors?

Similar to a solar panel’s STC and PTC ratings, batteries can also be subject to a lower kWh production when deployed outside the ideal conditions of a test laboratory. In addition to different temperatures, your battery’s actual $/kWh will also depend on its usage profile, recommended depth of discharge, power-to-energy ratio, calendar life, and other factors that an engineer should evaluate. Therefore, when calculating $/kWh, the kWh figure in the denominator needs to be corrected for the actual usable energy of the battery, not the nameplate.

For example, a 100 kWh battery that’s limited to a recommended 80% DoD will need to be calculated as 80 kWh. Additionally, your engineer should account for the battery’s 1-way efficiency, which could be as low as 86% in California, according to the 2016 Self Generation Incentive Program (SGIP) iTron report of average lithium-ion RTEs.

In this 100 kWh example, for the nameplate rating price of $500/kWh for a lithium-ion battery system in California, we could use the following formula to reveal the adjusted $/kWh that accounts for usable kWh energy for a $500/kWh quoted price:

$500/(80/100 x 86/100) ≈ $725/kWh … a 45% price increase!

Consequently, when someone quotes you a price in $/kWh, always ask the above questions, or at least ask if the price is calculated to include discounts for RTE and DoD. If their answer is, “Yes, we absolutely took that into account in that pricing,” then you’re set to compare apples-to-apples pricing. If they aren’t sure what you mean, or it’s not showing up in any purchase agreements or POs, then that’s a good indicator you need to double-check the fine print and perhaps do your own calculations or consult with an engineer to evaluate all the factors in your battery choice.

Question 4: What’s your levelized cost of energy (LCOE)?

As we’ve seen, regardless of whether the $/kWh number offered is in AC or DC watt-hours, or defined as OpEx or CapEx for tax purposes, the total cost of ownership picture is still incomplete.

As with total dollars-per-kilowatt solar PV system pricing, you’ll want your engineers to evaluate the total turnkey price that includes everything needed to be placed in service. In addition to the cost of the actual energy storage hardware, your LCOE figure will need to account for the cost of labor, engineering and feasibility studies, permits, utility interconnection applications and field studies, protection relays, total kWh throughput (including annual degradation of capacity and RTE), etc.

As long as your engineer has accounted for the above kWh questions and typical project costs, you’ll at least be able to make a somewhat-simple price comparison to evaluate your energy storage system. Of course, if you’d like the help of SepiSolar’s “solar enginerds” to evaluate your energy storage choices, please contact us.

To read Part 2 of our series about evaluating technology, applications and bankability, sign up for SepiSolar’s newsletter, or follow us on LinkedIn, Twitter or Facebook.

Josh Weiner is CEO and Founder of SepiSolar. Follow Josh on Twitter at @SepiSolarJosh

Josh Weiner


March 15, 2018 0

Solar engineers love a good mystery to solve. A few months ago, our engineers overheard a phone call between one of our team members and a customer who was on a job site in Oregon. They were discussing the utility service and voltage of some very old, unlabeled service equipment. With zero photos from the site and not even so much as a voltmeter available to the field tech, we wrote down everything the field tech said, which amounted to a rather short description:

There are two lines coming in from the pole, then a big transformer and a small transformer. I can’t read the voltages, but then three lines go out to the main disconnect.

At this point, most engineering companies would dig their heels in and insist on knowing the line-to-line as well as line-to-neutral voltages (among other things) before proceeding with the designing. But SepiSolar didn’t balk; we jogged our brains together while sketching on the white board and searching the web, but were initially stumped.

But it was something about the different sizes of the transformers that stuck with us; we knew that was the key data point. In about 10 minutes, we were confident the site was utilizing an ‘Open Delta’ service from the grid, which taps only two phases of the utility grid but delivers a three-phase circuit to the facility. Knowing that a service like this is meant to handle most of the power demands on the two phases that come from the larger transformer, we opted to interconnect all the PV power to these two phases, leaving the power leg and smaller transformer alone. Solar engineering mystery solved.

Solving engineering and design mysteries and optimizing the system for a unique site like this is exactly the kind of challenge that SepiSolar’s engineers thrive on. In under an hour, we had a full system design for this customer and explained to him exactly how he needs to install this project safely.

If you’ve got a solar mystery that needs solving, call a SepiSolar “enginerd.” We’d love to solve it for you.

Click here to learn more about our solar consulting services.

Josh Weiner

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