This article was originally published on July 20, 2022. It was updated and republished on September 12, 2023, to reflect new information. To navigate directly to the updated content, click here

As more and more rooftop solar, home batteries, and other distributed energy resources (DERs) are added to the grid, the requirements to ensure the safety and reliability of the electric grid are changing. Smart inverters are powerful tools for this purpose. They can sense conditions on the electric grid around them and respond intelligently, adjusting the amount and characteristics of the power sent to the grid by the solar panels or other DER they are connected to. In this way, they can help maintain the stability of the grid. 

It’s for that reason that a number of states leading the way on the renewable energy transition have decided to require smart inverters for new DER projects that seek to connect to the grid. However, given the complexity of these devices and the high stakes (keeping the grid running reliably!), its essential that there be a set of standardized requirements for these “grid-support” smart inverter functions (IEEE Standard 1547™-2018) and a rigorous and standardized process for testing these products to ensure the requirements are met (UL 1741 and related supplemental documents). 

Developing both of those standards required a significant amount of work by engineers and DER stakeholders and was an intensive multi-year process. While that process is now complete, smart inverters that have passed the necessary tests and are formally certified to 1547-2018 are not yet widely available on the market. Additionally, there remains considerable uncertainty about when they will be available to customers—making it very challenging for states to determine when smart inverter requirements for DERs should officially go into effect. 

For that reason, IREC undertook independent original research to gather data that could help determine when compliant smart inverters will reach the market, surveying key players from inverter manufacturers to the testing labs that will certify their products. We now have the first-ever estimate of this timeline, though a number of uncertainties remain—from the capacities of testing labs to continuing supply chain issues. In this article, we explore the methodology of our research, underlying assumptions, and the resulting estimated timelines for when smart inverters with grid-support capabilities will be available on the market. 


Fifteen years after the original IEEE 1547-2003 was published, the updated IEEE 1547-2018 established standards for greatly increased DER capabilities in terms of voltage and frequency ride-through, active and reactive power control, and monitoring and control. The accompanying test procedures of IEEE 1547.1, which specify how product compliance with the standard can be verified, were published in May 2020. 

Another necessary step to getting these products to market was to update the safety standard UL 1741; solar PV and energy storage inverters and some other products are listed to this standard, which requires grid-interactive equipment to pass the tests in IEEE 1547.1. As these greatly increased capabilities were desired in states planning for or already seeing high DER penetration, Underwriters Laboratories (UL) and the DER inverter industry planned to update the UL 1741 standard as soon as possible to expedite when compliant inverters could move to market. In August 2020, UL introduced “Supplement SB,” a three-page addition to the standard, which required testing of inverters to IEEE 1547.1-2020.

As Nationally Recognized Testing Laboratories (NRTLs) and manufacturers began preparing for certification testing, however, many questions arose about how to properly manage some aspects of the tests. It became clear that IEEE 1547.1-2020 had several areas that needed clarification. Otherwise, different products from different manufacturers, tested with different NRTLs, could present different responses to similar conditions. To achieve harmonization of responses between certified products, the UL 1741 Standards Technical Panel modified Supplement SB with the needed clarifications. 

The panel expended much effort in late 2020 and throughout 2021, creating twenty-eight pages worth of these clarifications and shepherding them through the UL balloting process. 1 The updates were eventually published on September 28, 2021 in the form of a new Third Edition of UL 1741. This change in edition allows utilities or other stakeholders to easily validate that a certification was carried out with the additional guidance. 

Several states, such as Hawaii (HECO) and Maryland, had planned to implement IEEE 1547-2018 requirements for interconnection applications starting January 1, 2022, about eighteen months after the publication of IEEE 1547.1-2020. This timeline was in line with previous changes to UL 1741, which typically included an eighteen-month grace period after which manufacturers were required to utilize changes to the standard. 

Due to the more than year-long delay in finalizing the testing requirements, however, it became obvious that inverters would not be able to be certified to the standard by that time. The question of when products would actually be available remained.

Due to the breadth of new testing requirements, and the fact that most manufacturers would likely be submitting products to be certified or re-certified within a similar timeframe, neither manufacturers nor NRTLs were sure when products could get to market. Throw in the challenges of the global pandemic and logistics and parts-sourcing challenges, and it was tough for anyone to guess.  

Gathering Data to Estimate the Timeline for Grid-Support Inverter Availability

In the summer of 2021, IREC determined that data was needed from manufacturers and NRTLs to determine how large of a testing effort would be involved to get all inverters certified to UL 1741 Third Edition Supplement SB, and thus when compliant products could be expected. Via surveys, IREC collected data from inverter manufacturers and NRTLs. 

IREC surveyed 11 inverter manufacturers about their intentions for certifying inverter families of different sizes. When testing for certification, one representative model can represent a “family,” which can consist of multiple models with different power outputs. IREC obtained more limited but useful survey data from two other manufacturers, bringing the total to 95 known inverter families to be certified. Given that many inverter manufacturers did not respond to the survey (approximately 20 others), there remained a fairly large number of unknown inverters to be certified. 

The extensive listing of UL 1741 SA2-certified inverters provided by the California Energy Commission (CEC)3 was used to add more data to the survey. For these non-surveyed manufacturers, IREC had to make assumptions about the models from each inverter manufacturer to group them into a smaller number of inverter families. The grouping was based on power output and other descriptive details provided in the CEC list. IREC assumed that each active manufacturer with equipment on the list would be seeking UL 1741 SB certification for that inverter with the same NRTL as was used for UL 1741 SA certification. This resulted in 60 more inverter families that were divided up amongst the three testing pathways for the respective NRTLs, for a total of 155 inverter families to be tested.

A separate survey collected data from three major NRTLs about their capabilities to support certifications through three different pathways. Products can go through one of three “testing pathways,” with varying degrees of effort required of NRTL engineers: 

1) testing at the NRTL’s laboratory, 

2) witness testing, where a NRTL engineer witnesses tests performed at another facility (often the manufacturer’s own test lab), or 

3) supervised testing, where the manufacturer is approved by the NRTL to run the tests itself, with the NRTL engineer providing a supervisory role. 

In any of these test scenarios, the NRTL processes the results to ensure conformance to the standard and, once tests are complete, can generally issue a certificate of compliance within a week or two. Testing capabilities for one major NRTL and two minor NRTLs who did not supply survey answers were assumed based on responses by other NRTLs. 

Interpreting the Survey Results

Based on feedback from manufacturers and NRTLs through the survey and other forums, IREC determined that the likely duration of each test (time from beginning setup to completion of the certification tests) for a single inverter family could range from nine to twelve weeks. The wide range in estimated duration was due to the understanding that this was a new testing regime, with more test runs than ever before required, and there would likely be a learning curve for both the manufacturer and NRTL test engineers. 

Knowing the total number of inverter families to be certified via each NRTL testing pathway, the number of tests that could run in parallel for each pathway, and the presumed duration of each test, IREC calculated a total time to complete each NRTL pathway. We assume all tests within a pathway are run back-to-back without lag between them. With a nine-week average test duration, about 85% of the total inverter families (131 out of 155) could be tested within about one year (54 weeks). With a twelve-week average test duration, about 85% of the total could be tested within about 16 or 17 months (72 weeks). One major NRTL reported very low parallel test capabilities, pushing completion of their tests out over two years. 

Cumulative certifications for each NRTL testing pathway over time. Vertical line represents one year.
Cumulative certifications for each NRTL testing pathway over time. Vertical line represents one year.

As mentioned, a certificate can be issued by the NRTL within a couple weeks of test completion, and then production of those units can begin. Getting those certified units into the hands of developers who can install them in the field can take more months. Inverters may be produced overseas and are typically transported to the U.S. by cargo ship, and then distribution must occur within the country. In earlier times, this process could take up to three months but the uncertainties of the global pandemic, as well as current supply chain and logistics issues, could have further impact. 

Based on discussions with manufacturers, it is likely reasonable to add approximately sixteen weeks from certificate issuance to receipt of inverters by project developers or distributors, with the caveat that the global situation will determine the speed of delivery. This results in a range of 16 to 21 months (72 to 90 weeks) from the start of testing to delivery. If we count from November 1, 2021 (discussed in the next section), that translates to deliveries from most manufacturers arriving by March to August of 2023. 

Major Uncertainties

In addition to the uncertainty introduced by the assumptions of NRTL capabilities and certification needs of non-surveyed manufacturers (noted above), the data contains other sources of uncertainty. It must be noted that while UL 1741 SB Third Edition was published at the end of September 2021, testing didn’t likely commence immediately or uniformly as manufacturers and NRTLs ramped up to begin the first tests. Therefore, the exact starting date to measure from is a little unclear, but November 1, 2021 can be chosen if we assume that it took about a month to get started. A small number of manufacturers may have begun testing before the standard was published, either with a draft or the understanding that additional testing based on clarifications in the final publication might be necessary. This may have allowed some of the inverter families certified in the early part of 2022 to beat others to market. 

Additional uncertainty comes from the fact that testing is not likely to be as uniform as calculated for this analysis. Here, we assume all inverter families take the same amount of time to test, and that each test can immediately follow the previous one. Unexpected results can and do sometimes occur in the middle of certification testing, requiring manufacturer engineers to change firmware and retest to gain compliance. Also, the number of tests run in parallel may not always be exactly the same as the capabilities as stated by the NRTLs, since test engineers may be working on multiple tests at once and delays could impact their ability to keep all tests running perfectly in parallel. Delays could be caused by either testing issues or personnel/staffing issues, including COVID-19 absences.

Adoption Developments

Some jurisdictions set required dates for specification of UL 1741 SB-certified inverters into interconnection requirements before the extent of inverter certification delays was known. The timelines were typically predicated on the date of the interconnection application, such that the customer would need to have a certified inverter model ready to identify in their application. 

For instance, Maryland and Washington D.C. both initially specified January 1, 2022 as the implementation date for certification. Washington D.C.’s interconnection rule (Title 15 Chapter 40) requires this change “upon commercial availability,” and thus implementation has been delayed until sufficient commercial availability can be determined.4 Meanwhile, Maryland’s Public Service Commission waived its timeline requirements (in COMAR indefinitely, to determine a definite timeline at a later date.5 Minnesota was the first state to incorporate IEEE 1547-2018 into its interconnection requirements; instead of setting a certain date for compliance, the Public Utility Commission opted to wait to rule on implementation until certified inverters are commercially available.6

Hawaiian Electric (the electric utility that serves most of the main Hawaiian islands, besides Kauai) initially aimed for April 1, 2022, but now has set the target date as October 1, 2022.7 This will likely be the earliest implementation in the U.S. Both Massachusetts and New York utilities were initially targeting mid-summer 2022, but have shifted to January 1, 2023, due to the certification delays.8 California also originally intended to require compliance in 2022, but has shifted to April 1, 2023 (eighteen months from publication of UL 1741 SB).9

While several jurisdictions are selecting specific dates for implementation of the new certification requirements, the uncertainties described above could potentially push a reasonable effective date out even further than April 1, 2023. On the other hand, if things go better than predicted at the NRTLs and with the global situation, there’s a chance an earlier date could be accommodated.

Update on Inverter Certifications as of September 2023

When this blog post was initially published in July 2022, IREC was aware of only three manufacturers that had concluded certification testing, accounting for five total inverter families of the 155 likely to be tested, or about 3%. This was considerably lower than the 12% predicted by our survey results using a twelve-week average test duration. We hypothesized that either the start date or test duration might need to be modified to align expectations. Given the small sample size at that time, we expected to have a better idea of certification progress over time.

Since then, IREC has continued to track the inverter families that have been certified to UL 1741 SB, using data both directly from inverter manufacturers as well as interpreting the California Energy Commission’s Grid Support Solar and Battery Inverter list. Unfortunately, it seems that a 12-week test time per inverter family may have been too short of a period to assume. Perhaps other delays prevented manufacturers and NRTLs from commencing testing or prevented back-to-back testing, or other hurdles arose as mentioned in the Major Uncertainties section above. 

Below, we compare the actual certifications that have been completed over time to the expected number of certifications based on an 18-week test time (our latest estimate based on the testing times observed to date). For simplicity, we begin the timeline on January 1, 2022. For much of the last year, actual certifications have lagged behind forecasted rollout. A recent uptick has brought actual certifications more in line with the expected level. 

The recent increase may indicate that manufacturers and labs are catching up and testing is starting to take less time per inverter family. This may mean that the actual certifications will at least match or exceed the “expected” line. If that holds true, we expect that at least 85% of inverters should be certified by the end of January 2024. As noted in the Interpreting the Survey Results section above, it may take more than sixteen weeks for inverters to reach the market after certification is achieved. 

It is worth noting that there seem to be quite a few new market entrants that weren’t accounted for in our  initial total. Therefore, the percentages in the “actual” line may be a little low compared to the actual total number of inverter families we’ll see on the market when all is said and done (i.e., there are likely to be more than 155 families). We can expect approximately the same number of inverters available, but they may be from different manufacturers than developers are used to using or procuring. That may mean that there’s still potential for market disruption if developers are forced to procure inverter models that are less well established in the market. 

Also of note, only 18 solar and storage inverter models greater than 250 kW are currently SB-certified per the CEC equipment list (approximately 11 families). Because the number of larger SB-certified inverters available is significantly lower compared to smaller inverters, efforts to acquire larger SB-certified equipment should consider the scarcity of such devices and potential competition in procurement. Additionally, only 7 of those larger models note Common Smart Inverter Profile (CSIP) conformance, which is a requirement for use in California.

Update on State Adoption

Some states shifted the rollout for requirements of SB-certified inverters due to the lack of available certified equipment. Hawaiian Electric transitioned to requiring SB-certified inverters in February 2023, and has not noted any issues with the rollout for its mostly rooftop PV market. California has once again shifted the compliance date to August 29, 2023, per supplemental advice letters filed by the state’s large utilities in May 2023 and approved by the CPUC on July 25th. Lack of larger certified inverters (e.g., over 250 kW) was one reason cited for the delay. Massachusetts started requiring SB-certified inverters for systems greater than 500 kW on January 1st of this year, though applications are accepted before certification is complete. Massachusetts utilities will require all applications, for all size systems, to choose SB-certified inverters on October 1st, 2023.


  1. In this process, the Standards Technical Panel (made up of a mix of various stakeholder segments) reviews proposals for updating the standard and votes whether or not to approve and/or modify them. Voting/commenting and comment resolution may be repeated for multiple rounds to achieve consensus before finalization and publication. Each round of balloting can last several months.
  2. The California Investor-Owned Utilities have required “smart inverters” per the testing requirements of UL 1741 SA since 2017 (see PG&E Advice Letter 4919-E Modifications to PG&E’s Electric Rule 21 Tariff Pursuant to D.14-12-035 to Reflect the Approval Date of Inverter Based Technologies Requirements, p.2-3 (September 13, 2016), These inverters have very similar capabilities compared to UL 1741 SB and the requirements of IEEE 1547-2018.
  5. Order No. 89933, Order on Recommendations of Interconnection Workgroup (September 9, 2021)
  6. MN Pub. Util. Comm., Dkts. E999/CI-16-521, E999/CI-01-1023, In the Matter of Updating the Generic Standards for the Interconnection and Operation of Distributed Generation Facilities Established Under Minn. Stat. §216B.1611, In the Matter of Establishing Generic Standards for Utility Tariffs for Interconnection and Operation of Distributed Generation Facilities under Minnesota Laws 2001, Chapter 212, Order Establishing Updated Technical Interconnection and Interoperability Requirements, p. 11 (Jan. 22, 2020),{80F9CE6F-0000-C13C-96FB-CB223509EEE9}&documentTitle=20201-159427-02
  9. Southern California Edison, Advice Letter 4824-E, Modifications to Electric Tariff Rule 21 to Incorporate IEEE 1547.1-2020 Test Procedures into Testing Regime for Phase 2 and 3 Requirements in Compliance with Resolutions E-5000 and E-5036, p. 4 (July 1, 2022),