800 VA or 800 W? What the plug-in solar limit actually means

Two documents set the ceiling for plug-in solar in Great Britain, and they use different units. The statutory definition in SI 2026/848 says a plug-in microgenerator has a maximum rated AC output not exceeding 800 watts. The Interim Product Specification sets a maximum apparent power, Smax, of 800 VA, measured as the highest ten-minute average.

For most devices, on most days, those two figures describe the same thing. They come apart at non-unity power factor, which is why the limit is best written as 800 VA / 800 W. The same difference explains why a 2,000 W array behind an 800 W inverter is entirely legitimate.

Watts, volt-amps and the bit in between

A watt measures real power: energy actually delivered and turned into heat, light or motion. Your meter bills you in kilowatt-hours, which is real power over time.

A volt-amp measures apparent power, the voltage multiplied by the current. In an AC circuit, voltage and current rise and fall as waves, and they do not always peak together. Where they are out of step, some of the current sloshing back and forth in the wires does no useful work but still occupies the cable, the plug and the socket. Apparent power counts all of that current. Real power counts only the part doing work.

The ratio of the two is the power factor. At a power factor of 1.0 the waves are in step and 800 VA is 800 W. At 0.95, a realistic figure for a small grid-tied inverter, 800 VA corresponds to roughly 760 W of real power. The apparent power is the larger number, so a device capped at 800 VA can never exceed 800 W of real output, while a device capped at 800 W of real output could in principle draw more than 800 VA of current.

The 800 VA limit is the safer of the two

Conductors, fuses and contacts are heated by current, and current is what apparent power tracks. Limiting VA limits the current that the plug, the socket and the final circuit have to carry, regardless of how much of it is doing useful work. That is what you want when the connection point is an ordinary domestic socket rather than a dedicated circuit.

Read the two documents together and the practical position is straightforward: build to 800 VA and you satisfy both, because you cannot exceed 800 W of real power while staying inside 800 VA of apparent power.

The ten-minute average, and why it is not a loophole

Smax is defined as the highest ten-minute average rather than an instantaneous peak. It sounds permissive, but two things close it off.

First, it matches how thermal limits actually work. Heat takes time to build in a plug or a length of cable, so what damages a connection is sustained current over minutes rather than a half-second spike. A ten-minute averaging window is a reasonable proxy for the timescale on which a connection warms up.

Second, the specification imposes a hard instantaneous ceiling alongside it: a maximum of 3.5 A AC at the point of connection. At a nominal 230 V, 3.5 A is around 805 VA, so the current limit and the power limit describe the same envelope from two directions. There is no window in which a device can sit at, say, 2 kW for two minutes and average itself back down.

The separate DC ceiling: 2,000 W of panel

Distinct from the AC limit, the specification caps the DC side at 2,000 W of total module Pmax, with a maximum of 120 V open-circuit at the inverter inputs.

The voltage limit is about touch safety and the practicalities of DC in a domestic setting. At 120 V open-circuit the array stays well below the levels a rooftop string system runs at. The 2,000 W figure matters more in practice, because it explicitly permits something that looks, at first glance, like getting round the 800 W rule.

Why over-panelling is standard practice

Fitting more panel capacity than inverter capacity is normal across the whole solar industry, and it is sensible here for the same reasons it is sensible on a roof.

A panel’s Pmax is its output under standard test conditions: 1,000 W per square metre of irradiance at a cell temperature of 25 °C. British conditions rarely oblige. Panels get hotter than 25 °C in summer, which reduces output, and full irradiance in this country is the exception. Most generating hours are hazy, overcast, early or late.

Balcony and wall-mounted arrays make this sharper still. A vertical south-facing panel loses a good deal relative to an optimally tilted one. An east or west aspect produces a broad, low curve rather than a tall midday peak. In all those cases an 800 W inverter fed by 800 W of panel spends nearly all its life running well below capacity.

Put 1,400 or 1,600 W of module on the same inverter and the mornings, the evenings and the grey days lift substantially, while the handful of bright midday hours are clipped at 800 VA. Since the inverter enforces the cap, the network sees no more than it ever would. You have widened the part of the day in which the device does useful work. Given that self-consumption is the whole economic argument for plug-in solar, spreading generation across the day is worth more than a brief peak you would probably export for nothing.

The limits on over-panelling

• Total module Pmax must not exceed 2,000 W, and open-circuit voltage at the inverter inputs must not exceed 120 V.
• The inverter must be capable of the DC input you are giving it. The legal ceiling is a different figure from the manufacturer’s rating, and the manufacturer’s rating governs the warranty.
• Returns diminish. Going from 800 W to about 1,600 W of module buys a lot. The last few hundred watts buy progressively less, and each panel costs the same.
• Physical space usually bites first. Two panels are a realistic balcony installation, and four is a wall or a garden.

What the cap means for output and savings

An 800 W array facing south in central England produces roughly 650 to 760 kWh a year on PVGIS modelling. What you keep of that depends almost entirely on self-consumption: roughly 30% to 35% for a household out all day with no storage, and 70% to 90% for someone at home during the day. At around 25p per unit, that is broadly £50 to £60 a year at the low end and £150 to £190 at the high end.

You are very unlikely to be paid for exports, because plug-in solar sits outside MCS and the Smart Export Guarantee requires MCS certification and an export-recording smart meter. The value is therefore in what you use at the moment it is generated, which is the real argument for over-panelling within the DC ceiling: it buys more generating hours. The arithmetic is worked through in our savings breakdown.

Two things the 800 VA cap does not do

It does not let you install several devices to reach a larger total. G98 Issue 2 Amendment 1 (2026) permits one device per household, and that is the operative rule. The specification anticipates a move to one per final ring circuit and the government has said it intends to go that way, but only subject to an amendment to G98 that has not been made and has no date. See the per-household rule.

It does not exempt you from notification either. There is no small-generator carve-out: connection falls under G98, and the installer must advise the distribution network operator no later than 28 days after commissioning, inclusive of the commissioning day, using the Form B Installation Document.

For the rest of the framework, see what became legal on 27 August 2026 and the worth-it guide.