February 20, 2019

When it comes to choosing the equipment used to interconnect the combined output of a PV system to the utility grid, it is important to ensure that the equipment is sized in order to withstand the full amount of power capable of being delivered by the PV system.

SepiSolar’s recently published a white paper that describes what our engineers consider before recommending PV system output equipment.

Download this resource and learn how SepiSolar engineers conduct a comprehensive evaluation.


February 14, 2019

If you’re ever in the market to buy or sell an existing solar project, you’ll want to have an independent solar engineer to technically review and evaluate those assets.

SepiSolar’s recently published a white paper that describes our solar PV evaluation procedure. This PDF download also includes a case study of our evaluation of a 409 kW rooftop solar project.

Download this resource and learn how SepiSolar engineers conduct a comprehensive electrical and structural evaluation.


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

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, 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


June 20, 2018 0

If you missed our first Ask SepiSolar Anything with SepiSolar’s CEO Josh Weiner answering solar+energy storage questions, you can’t ask any more questions, but you can listen to the whole session here:

Josh tackled some great questions from people tuning in to this live and interactive program:

  1. What are some of the common KW inverter sizes for C&I solar+storage? (at 11:12)
  2. When you’re doing solar PV plus storage, what kind of interconnection requirements, like 120 percent rule, do people face? Are you seeing more AC or DC coupling on the system? (at 13:20).
  3. For commercial solar+storage, who are the customers that would benefit the most from solar+storage, and who are the customers that aren’t yet ready? (at 20:40)
  4. What type of energy storage is available and viable for the Florida residential and commercial market? Which brands? Also, does storage benefit from the 30% ITC credit if installed in conjunction with solar? (at 26:00)
  5. For O&M requirements for lithium-ion batteries for C&I solar+storage applications, is there maintenance required on a yearly basis? Or is it bi-yearly? What kind of components need to be serviced? What kind need to be replaced? (at 30:15)
  6. Is there an unbiased accurate chart from an independent testing lab of expected life cycles of all that all the battery brands? (Short answer, yes, but there’s only one that’s public.) (at 38:02)
  7. How does energy solar PV and energy storage work with virtual net metering and aggregate net metering? (at 43:22)
  8. What is the unit based cost for battery O&M? (at 47:10)

Join us for our next Ask SepiSolar Anything

If you have more questions about solar+storage, or energy storage or anything related to solar, join us for our July edition of Ask SepiSolar Anything. You can be in the audience and ask your questions live at Intersolar North America or tune in virtually.

Topic: Ask SepiSolar Anything about energy storage technologies. Josh will be answering questions with our special guest, Matt Harper, Chief Product Officer of Avalon Battery.

When: Thursday, July 12, at 1pm Pacific.

Where: Sign up to get a link to watch via the web or join us live at Intersolar. Get all the info and a reminder here!

P.S. If you’re on Twitter and want to meet other solar people behind the solar brands on Twitter, RSVP for the 8th Annual Intersolar Tweetup, which @SepiSolar is sponsoring. Space is limited.


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.


September 12, 2017 0

Ok, let’s cut to the chase. Here are the new Code sections that Bill Brooks and team have implemented as part of the new 2017 cycle and why they’re awesome:

NEC 691 – Large-Scale Photovoltaic (PV) Electric Power Production Facility

We finally have a section that addresses large-scale PV! Before this, all we had was 690.80 for systems over 1000 V, which just redirects us to generic ol’ NEC 490, which is for any equipment over 1000 V, not just photovoltaic (PV). Ok, we also had 705.12(C), but, admit it, that was a fairly useless section. Now, at least, we have a dedicated section for large-scale PV that will undoubtedly grow and evolve over time.

This section was not overdone. When introducing a new chapter, it’s very easy for the authors to go a bit nuts and add a lot of rules from the get-go without understanding how the article will be adopted, enforced, and exercised in the real world. I applaud the balance that the CMP crew brought to this particular section.

NEC 705 – Interconnected Electric Power Production Sources

New definitions! We now have definitions for microgrid systems and microgrid interconnects! The common message with the new 2017 NEC is definitions, definitions, and definitions. Even with old language (like grounding, where the phrase “functional grounding” is now being used), this NEC version is making a clear directive to be clear in how we talk and address these things. I, for one, am really thankful for the clarity this Code cycle brings to our industry.

120% rule on center fed panels. Remember how painful it was when 690.64(B), which later became 705.12(D), revised their rules to eliminate the 120% allowance for center-fed panels? Then, remember how Code officials allowed us to use the 120% rule but only if detailed busbar load calculations were provided? As much as I like to earn solar engineering business, I definitely do not believe in arbitrarily creating additional engineering and design work for industry professionals. The name of the game is to lower costs and accelerate system installations, without sacrificing safety. Well, let’s give a big shout out to the CMP panel because all those rules are now gone, and we can go back to the good ol’ days of using the 120% rule for center-fed panels. Be sure to check out the new 705.12(B)(2)(3)(d).

Available fault current and busbar load calculations remain. Sometimes when Code officials revert or “undo” certain code sections like the one above about the 120% rule for center-fed panels, they also undo some of the new code requirements that came along with those. Not this time. If you want to venture outside the industry standard and work on more complex interconnections, we still can perform detailed busbar calculations and fault current analysis to ensure that your (complex, unique) system is designed to Code.

NEC 706 – Energy Storage Systems

It’s finally here! I’m just so glad we finally have a section dedicated to energy storage. Sometimes I almost don’t even care what’s in it, just as long as it’s there, but this section goes above-and-beyond my wildest expectations for a great release of the first generation of an energy storage system-dedicated code section.

Definitions. Just read the definitions section. It’s a great read. Let’s all use this language to create clarity in an otherwise young, unclear, and ever-developing marketplace. Be sure to read the 3 italicized definitions in particular:

  • Energy Storage System, Self-Contained
  • Energy Storage System, Pre-Engineered of Matched Components
  • Energy Storage System, Other

It’s really important to understand the differences between these, since battery systems come together in the field in a variety of different ways, and if you want to fly through permitting and inspection processes, I suggest using the correct language for the correct system to help push projects through more quickly. For instance, UL 9540 is a standard now, let’s use it! Be sure to work with manufacturers that have this certification as it makes the entire process easier. Yes, UL 1642, 1973, and 1741 SA are all great standards to refer to, but nothing is quite as nice as having a full wrap of 9540 on all components.

Battery locations, terminations, and types, oh my! I think what I like most about this section is the care that the CMP took to address as many technology types and unique requirements as possible. For instance, the worst thing that can happen to a lead-acid or lithium battery is that it could explode or catch fire under short-circuit conditions. However, the worst thing that can happen to a flow battery is that it will leak and cause rusting of metal components. This code section attempts to address these unique concerns differently, for each technology, which is to say the Code officials are considering the uniqueness and specific applicability of these types of technologies and products. I like detail work!

NEC 710 – Stand-Alone Systems

I wouldn’t necessarily call this section revolutionary, but it is nice to finally have a standard. We’ve been designing and installing off-grid PV and energy storage systems for decades, and this is the first time there is a dedicated section to address those installations with some basic standards. This is a quick read, and it ought to confirm to anyone who’s designed or installed an off-grid system that this is how it ought to be done (at a minimum). I’m curious to see how this section develops and evolves over time. For instance, the PV must not be sized to handle 100% of load, batteries need not be included, etc. These are just some of the nice things to have in a Code that we can fall back on in the presence of inspectors or plan checkers.

Don’t worry, 690.56(A) and 706.11(B) still apply.

NEC 712 – Direct Current Microgrids

Again, read the definitions. I know I’m starting to sound like a broken record, but before we can agree on rules, costs, and benefits of these systems, we really need to understand how to talk about them. Here’s to assimilating code language in everyday work life!

Finally, we have a definition of system voltage. With DC micro-grids, it’s not uncommon to have 5 different voltages: low-voltage controls / sensing, high voltage power, DC-to-AC inversion, auxiliary loads, etc. Now we have a definition of what the entire system voltage ought to be. Read 712.30 to learn more. Same goes for available DC short-circuit current (712.65), OCPD (712.70), so I highly recommend reading this section. It’s only 2 pages, so it’ll be a breeze.

Thank you for reading, and as always, don’t hesitate to reach out to SepiSolar if you’d like to learn more about how to integrate these types of systems that can involve PV, energy storage, generators and grid interconnections, or for information about our solar consulting services. There are NEC codes coming out, UL standards, new product configurations, and new technologies entering the market, and as an independent, 3rd party technical consulting firm, we can provide unique perspectives on the state of these products, technologies, and applications to help ensure that your customers get the best experience and solutions possible. We’re here to help!

Visit this link for information about our solar consulting services for both residential and commercial projects.


August 3, 2017 0

August 2 was Earth Overshoot Day, and while it sounds like one of those quirky internet holidays, it is not one that our solar consulting services company celebrates. In fact, it’s a day no one will celebrate. It’s the date when the population’s demand for natural resources in a given year exceeds the planet’s capacity to replenish them. Formerly known as Ecological Debt Day, it marks the point when the yearly deficit truly begins.

The Concept

Earth Overshoot Day is calculated by the Global Footprint Network. This international group strives to changes how the world manages climate change and its ecological resources. They developed the Ecological Footprint, which measures the natural resources remaining on our planet against demand. To determine Earth Overshoot Day, they calculate how many days of the year that the Earth’s biocapacity can fulfill the population’s demand. The remaining number of days left during that year represents the overshoot.

Since Earth Overshoot Day fell on August 2, this means that every day until 2018, we are operating at a deficit of natural resources. This means that all the natural resources used from now until then are unsustainable. It’s a scary prospect considering we are barely halfway through the year.
The first Earth Overshoot Day was devised by Andrew Simms of the New Economics Foundation in London. In 2006, the think tank partnered with Global Footprint Network to launch the first campaign for the overshoot concept. That year, Earth Overshoot Day happened in October. If you do the math, you’ll find that in just over 10 years, we’ve already “lost” two months.

Contributing Factors

Much research shows that the three major factors affecting our dwindling natural resources include deforestation, overfishing and the emission of greenhouse gases. A way you can quickly contribute towards reducing this resource deficit is to go solar! Solar panels use no combustion and therefore emit no greenhouse gases to generate power. Going solar will reduce your carbon footprint and ultimately save you money on utilities.

Click through for information about our solar consulting services.


April 20, 2017 0

Saturday, April 22 is Earth Day. It’s the perfect time to consider going solar if you haven’t already. Solar energy was first harnessed

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