The 800 W limit: per circuit or per household?

Why there are two answers in circulation

Three documents, published within weeks of each other, each answer a slightly different question. Coverage that quotes one without the others produces a confident answer that is half right.

SI 2026/848 — the law. It defines a plug-in microgenerator as one that “has a maximum rated alternating current output not exceeding 800 watts”. This is a limit on the product. The instrument says nothing about how many you may own or where you may plug them.

The Interim Product Specification — what a compliant product must be. It caps the device at 800 VA of apparent power and 3.5 A, and requires manufacturers to “clearly state that only one plug-in solar device of up to 800 VA shall be installed per household circuit”, with the same statement on the product label. This is where “per circuit” comes from.

The government response — what the government actually decided, and the document almost nobody quotes:

the government has decided that the IPS should allow one plug-in solar device per individual power circuit protected by a circuit breaker at the consumer unit, subject to a corresponding amendment to G98. Until such an amendment is made, the current requirement in G98 limiting installations to one device per household will continue to apply.

So the sequence is: the law permits the product; the specification describes a per-circuit world; and an engineering recommendation that has not yet been amended holds the number at one per home in the meantime.

Watts and volt-amperes are not the same thing

The instrument says 800 watts. The specification says 800 VA. These are different quantities, and using them interchangeably is the tell of a page that has not read either document.

Watts measure real power — the power actually doing work. Volt-amperes measure apparent power, the product of voltage and current regardless of how well they are in phase. The ratio between them is the power factor. A domestic grid-following inverter runs close to unity power factor, so in practice the two numbers are nearly the same for these devices — but “nearly the same in practice” is not “the same”, and the two documents chose their units deliberately.

The specification also sets a current limit that is easy to overlook: a maximum of 3.5 A supplied to the mains installation.

What counts as a circuit

If the per-circuit allowance does come into force, the unit is not “a room” or “a socket” or “a ring main” loosely defined. The government’s wording is an “individual power circuit protected by a circuit breaker at the consumer unit”.

The specification includes its own recommended consumer guidance for working this out, and it is worth quoting because it is the practical test:

An electrical circuit typically supplies multiple sockets and is protected by a single circuit breaker in the consumer unit/fuse box. Sockets that are supplied by the same breaker are part of the same circuit. Users can identify which sockets are on the same circuit by observing which sockets lose power then the circuit breaker in the consumer unit/fuse box is switched off. Sockets that turn off together are on the same circuit.

Two conditions attach to that in the specification. Where users are unsure how to identify circuits safely, they “shall be advised to consult a professional electrician”. And users must be advised to check the installation has “modern residual current protection (RCBO) and is in good condition” — where it uses older fuse protection without RCBOs, it “shall be checked and, where necessary, upgraded by a professional electrician”.

The government’s reasoning for allowing more than one device at all was simply that “most UK dwellings have a minimum of two power circuits”.

Ring final circuits

A specifically British complication, since UK homes commonly wire sockets in a loop back to the consumer unit rather than on radial runs, and some respondents argued plug-in solar was unsuited to that. The government’s answer:

The electrical safety study published as part of this consultation assessed the operation of plug-in solar within representative UK installations, including ring circuits. The testing did not identify safety concerns that would prevent the safe operation of plug-in solar with a maximum rated output of 800W on appropriately protected ring circuits.

Note the qualifier — appropriately protected. That is the RCBO point above, not a throwaway.

What the limit does not cover

Panel capacity is a separate, larger number. The 800 VA cap is on what the inverter feeds into your wiring. The panels themselves may total up to 2000 W: “the maximum permissible sum of PV module DC power output (Pmax according to BS EN IEC 61730-1-2018) is 2000 W”. Above 960 W of panels, the manufacturer must advise you to consider a professional assessment of your electrical installation first.

Total capacity per dwelling was considered and rejected. Some respondents wanted a cap of 800 VA across the whole home regardless of circuits. The government’s view was that “the evidence for doing this is weak considering most UK dwellings have a minimum of two power circuits”.

Batteries are outside all of it. The instrument’s definition excludes a device designed to import electricity for storage, and the government response confirms battery-integrated products remain out of scope.

What we could not confirm

Whether the G98 amendment has been made. That single fact decides whether the answer to this page’s title is “per circuit” or “per household” today. The Energy Networks Association publishes current versions of G98 through its engineering database rather than as an open file, and we could not reach it. Our G98 page sets out exactly what we read and where it came from.

Until we can confirm it, we are giving the government’s own stated fallback — one per household — rather than the more permissive answer that is easier to write and better for anyone selling kits.