UL 3700 Explained: The Standard That Made Plug-In Solar Possible in America

For most of the last decade, Americans watching Germans hang solar panels off their balconies asked a reasonable question: if millions of these things work fine in Europe, why can I not buy one here?

The answer was not a ban. It was an absence.

The certification gap

American electrical safety runs on listed equipment. A product is tested by a Nationally Recognized Testing Laboratory against a published standard, and that listing is what a code official, an insurer, and a utility all rely on. If no standard covers what your product does, there is nothing to test against — and a device with no listing is a device nobody in the chain will approve.

Grid-tied inverters had a standard: UL 1741, harmonised with IEEE 1547 for interconnection behaviour. But UL 1741 assumes an inverter is hard-wired by a professional into a circuit engineered for it. It says nothing about the specific situation of a photovoltaic source connected to an ordinary branch circuit through a cord and a plug, installed by a homeowner, on wiring nobody surveyed first.

That gap was noticed the moment Utah began debating its plug-in solar bill in early 2025. Industry commentary at the time was blunt: UL listing for this intended application "is a lot more involved than meeting UL 1741," and no commercially available system met both UL and NEC requirements for it. Utah passed a law legalising a product category that, strictly speaking, did not yet exist in compliant form.

What UL 3700 is

UL Solutions published UL 3700, Outline of Investigation for Interactive Plug-In Photovoltaic Equipment and Systems, in December 2025, and announced the testing and certification framework publicly on January 8, 2026.

An Outline of Investigation is UL's mechanism for moving quickly: a documented set of criteria a lab can certify against while a full consensus standard works its way through committee. Functionally, for a manufacturer, it does the job.

UL 3700 sets criteria in three areas — construction, performance, and labelling — and it was written around four specific hazards:

1. Accidental contact with hazardous electrical components. A wall plug was designed as a load connector. UL notes ordinary plugs "have not been evaluated as power sources," so a plug-in PV device must ensure its pins are not live when unplugged.
2. Safe installation. Because the installer is usually the homeowner, instructions and labelling carry safety weight they would not in a professionally installed system.
3. Protection against system overloads. This is the breaker-masking problem: injected current can hide a downstream overload from a breaker that only senses its own end of the circuit. The design literature points to dedicated circuits, solar-specific receptacles, or connection to circuits with oversized conductors as mitigations.
4. Prevention of reverse current flow into electrical lines. Anti-islanding — the system must not energise a dead grid.

Ken Boyce, VP of principal engineering at UL Solutions, framed the purpose directly: by defining how these systems must be built, evaluated and installed, the standard helps mitigate the risks.

The framework also draws on a UL white paper with the unglamorous but precisely accurate title Interactions of Plug-In PV (PIPV) with Protection of Existing Power Systems.

The first product built to it

In July 2026, roughly seven months after publication, Hoymiles announced the HiFlow Pro, presented as the first UL 3700-compliant plug-in microinverter available in the US.

One caution on wording, because it matters legally: Hoymiles' own release says compliant. Some coverage has reported it as certified. Those are different facts — Maryland and Colorado key their requirements to a listing by a national testing laboratory, not to a manufacturer's compliance claim. Ask any seller for the listing itself.

The specifications tell you a lot about how the standard shapes hardware:

• 360 W output per unit, parallelable to four units for 1,200 W — exactly the cap most state laws chose
• 99.8% static MPPT efficiency, 99.5% dynamic
• NEMA Type 6 enclosure, rated −40°F to 149°F
• Anti-islanding protection built in
• 4 lbs, 182 × 164 × 30 mm
• 250,000 hours of reliability testing behind it
• Rated annual production of 721 kWh for one unit

One oddity worth noting: Hoymiles' own four-unit figure of 2,403 kWh/year implies about 601 kWh per unit, not 721. Treat vendor production numbers as optimistic and model your own site instead.

The modular design is the interesting choice. Rather than shipping a single 1,200 W unit, Hoymiles built a 360 W building block that stacks. That lets a buyer start with one panel and scale, and it lets one product serve states with different caps.

Why this matters legally, not just technically

The standard would be a footnote if it were only an engineering document. It is not, because state legislatures wrote it — or its equivalent — into law.

Maryland's HB 1532 requires UL certification or equivalent, exempting only devices of 391 watts or less. Colorado's HB 26-1007 requires labelling or listing by a national testing laboratory, with the same 391 W exemption. California's SB 868 requires certification by UL or an equivalent lab. Utah's H.B. 340, drafted before the standard existed, is looser — but the pv magazine reporting on it noted compliance with NEC and UL certification as expectations.

So "is it UL 3700 listed?" has stopped being a nice-to-have question. In several states it is now the difference between a device you may legally connect and one you may not.

What to do with this as a buyer

Three practical implications.

Ask for the listing, not the claim. "UL certified components," "UL 3700 compliant," and "UL 3700 listed" are three different statements, and only the last one means a testing laboratory has issued a listing. A kit can also pair a listed panel with an uncertified inverter. Ask which standard, for which part, and who issued it.

UL 1741 SB is the acceptable fallback, for now. As of late 2026 the list is short but no longer a list of one. Alongside the Hoymiles HiFlow Pro, APsystems’ EZ1 plug-in microinverter is now CSA Certified to UL 3700 and sold in the US — and note the wording difference: a certification issued by a testing laboratory is a stronger claim than a manufacturer stating compliance. Ask which you are being offered. If your preferred kit is not yet listed to 3700, a UL 1741 SB microinverter from a reputable manufacturer is meaningfully better than an uncertified import, though it does not carry the plug-specific evaluation.

Expect the market to consolidate around it quickly. Once a standard exists and states start citing it, uncertified grey-market kits move from "unregulated" to "non-compliant." If you are buying to keep, buy the listed one.

The gap that kept plug-in solar out of America for a decade is now closed. What remains is the slower work of getting products through certification and getting the remaining forty-odd states to update their interconnection rules.

Sources

1. UL Solutions — UL 3700 announcement
2. Electrek — First UL 3700 balcony solar microinverter
3. pv magazine — Hoymiles introduces first UL 3700-compliant microinverter
4. pv magazine — How to design a UL-certified balcony solar kit
5. pv magazine USA — Balcony solar in Utah

Tools

• Check the law in your state
• Savings and payback calculator
• Which kit to buy

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