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


SepiSolar’s CEO Josh Weiner was recently interviewed by David Brearley in SolarPro Magazine’s In-House versus Outsourcing Solar Engineering Services in the May/June 2018 issues.

Not every solar company needs to be outsourcing solar engineering and design, but Josh comments on why quality design firms can be the most efficient and cost-effective solar service solutions for many solar contractors, developers, and EPCs.

SepiSolar engineers aren’t just NABCEP solar+storage design experts, they’re also requirements experts, producing designs that meet the detailed requirements of of any AHJ or utility across the U.S.

Read the Interview Here




March 6, 2018 0

The following blog post was contributed by CivicSolar’s Conor Walsh, Senior Account Manager at CivicSolar.



This article is a follow-up to a previous Civic Solar article discussing UL 1741 & Rule 21: Advanced Inverter Tests. We suggest you start with that article first.

Seemingly discontent to be left out of the advanced inverter functionality game, ISO New England (ISO-NE) recently announced new requirements for interconnected PV inverters in Massachusetts. Similar to California’s Rule 21, the new ISO-NE settings requirements are designed to maintain grid stability and resilience during both normal and abnormal operating conditions in the presence of a large volume of interconnected dynamic PV systems.

To achieve this mandate ISO-NE has promulgated a set of inverter ride-through requirements to be manually adjusted on UL 1741 SA  inverters that specify voltage and frequency trip settings and ride-through capability described in the ISO-NE Inverter Source Requirement Document. Implementation of these setting requirements vary by system size:

  • < 100kW: All inverter-based solar PV projects 100kW or less with applications submitted on or after June 1, 2018 are subject to ISO-NE Ride-through Requirements.
  • > 100kW: All inverter-based solar PV projects greater than 100KW with applications submitted on or after March 1, 2018 are subject to ISO-NE Ride-through Requirements.
    • Inverter-based solar PV projects with applications submitted prior to the above dates are encouraged to comply with ISO-NE Ride-through Requirements with the approval of the interconnecting utility.

Don’t fret. ISO-NE is actively engaging inverter manufacturers to ship units with a pre-programmed NE Regional Setting Group, that when selected, defaults to the required setting described in the ISO-NE Inverter Source Requirement Document.

What does this mean for your next PV project in the Commonwealth? First, be sure you are purchasing and installing UL 1741 SA compliant inverters. Below is a short list of compliant manufacturers. Secondly, begin to become comfortable with the ins-and-outs of the new setting requirements and how they are made. It may be several months until a default setting is available.

  • Enphase
  • Fronius
  • SMA
  • SolarEdge
  • Yaskawa – Solectria

For additional readings, please see below:

  1.  What is UL 1741 SA Advanced Inverter Testing and Rule 21?, UL Laboratories, 2016
  2.  2018 Regional Electricty Outlook, ISO New England, 2018

SepiSolar Editor’s note: While UL 1741 SA is a great standard that allows “smart” inverters to work dynamically with the utility grid, SepiSolar is excited about further developments in firmware architecture beyond UL 1741 SA. For instance, with upcoming new solar PV + storage systems, inverter firmware can be modified to allow PV to continue to stay on, beyond the sunset, or in spite of grid instability, without requiring ramping down the real power export of PV. Stay tuned.

If you’d like to contribute your thoughts about solar or storage design, engineering, codes, and standards, please send your articles to  

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