SI 2026 No. 848 caps a plug-in microgenerator at 800 watts of AC output. The Interim Product Specification that same instrument points to caps the same device at 800 VA. Watts and volt-amps don’t measure the same thing, neither document explains the difference, and a compliant kit has to satisfy both.
That’s the whole of it, and it matters more than a pedantic units argument usually would. From 27 August 2026 the UK plug-in route opens, and the number printed on the side of an inverter is one of the things a buyer is being asked to check. Two official documents give that number in two different units, and the retail badge on an 800W kit is a third thing again.
The discrepancy, stated plainly
The statutory instrument says 800 watts. The specification says 800 VA. Watts measure real power, volt-amps measure apparent power, and the two are only equal when the power factor is exactly 1. We’ve read the instrument, the specification and the government response, and none of them reconciles the two figures or acknowledges that they differ. We’re not going to pretend otherwise, and neither should anyone selling you a kit.
Where each figure comes from
The two numbers sit in two different places, and it’s worth being precise about which is which.
The statutory instrument. SI 2026 No. 848 was made on 16 July and comes into force on 27 August 2026. It defines a "plug-in microgenerator" in limb (b) as a source of energy which "has a maximum rated alternating current output not exceeding 800 watts". That definition appears twice, identically: once in regulation 2, which amends the Plugs and Sockets etc. (Safety) Regulations 1994, and once in regulation 3, which amends the Electricity Safety, Quality and Continuity Regulations 2002. The figure 800 appears in the instrument exactly twice, both times as "800 watts".
The specification. The Plug-in Solar Device Interim Product Specification, version 2.0, was published by DESNZ in July 2026. Its scope section limits the document to single-phase plug-in solar with "a maximum apparent power not exceeding 800 VA". Clause 4.1 repeats it as a design requirement: devices "shall be designed such that the maximum apparent power Smax supplied to the mains installation does not exceed 800 VA and the maximum current does not exceed 3.5 A". Clause 6.4.1 puts it on the inverter itself: "The inverter shall have a declared maximum apparent power not exceeding 800 VA." The specification’s own type-test table lists it as ≤ 800 VA. The word "watt" doesn’t appear anywhere in the document.
These aren’t alternatives you get to pick between. Regulation 3 of the SI says a plug-in microgenerator "must comply with the Plug-in Solar Device Interim Product Specification", naming version 2 published on 16 July 2026. So the definition gate is written in watts, and the specification it hands you is written in volt-amps. Our summary of what the specification actually requires covers the rest of its limits, and the wider picture of what becomes legal in August 2026 sits alongside it.
What VA and W actually measure
Here’s the physics, kept as short as it can honestly be kept.
Start with how the inverter works. Solar panels generate DC. The inverter converts the DC electricity coming off the solar panel array into AC power at mains voltage and frequency, then pushes it out through the plug. That DC to AC conversion is where both of these numbers get declared, and it’s the same job a full rooftop solar installation asks of a much bigger box.
Apparent power (VA) is voltage multiplied by current. It’s what the wiring, the plug, the fuse and the socket have to carry. If a device pushes 3.48 A into a 230 V supply, that’s 800 VA, whatever the current is doing relative to the voltage.
Real power (W) is the part of that which does useful work. It’s what shows up as heat in a kettle, cold in a fridge, or a unit of grid electricity you didn’t have to buy. Appliance power draw is quoted in watts, and so is your bill.
Power factor is the ratio between them: real power divided by apparent power, a number between 0 and 1. On AC, current and voltage don’t have to peak at the same instant. Where they drift apart, some of the current sloshes back and forth without ever delivering energy. That current still has to flow through the cable, and it still counts towards the VA figure, but it contributes nothing in watts.
The relationship is just W = VA × power factor. At a power factor of 1 the two are identical and the argument disappears. At 0.9 an 800 VA inverter delivers 720 W. At 0.8 it delivers 640 W. Because power factor can’t exceed 1, apparent power is always the larger of the two numbers, which is why the VA figure is the one that sizes cables and fuses.
This isn’t specific to plug-in solar. Anyone who’s had to size an inverter for an off-grid or 12V solar setup has met it, because inverters for campervans and boats are routinely sold with a W badge and a VA rating on the same page. The same gap shows up on string inverters, hybrid inverters and industrial UPS units, and in DIY solar discussion the two units tend to get used interchangeably. What’s unusual here is finding them in two official documents that are supposed to describe one limit.
Power factor is the whole of the gap
A grid-tied solar inverter in domestic solar service normally runs at a power factor very close to 1, because that’s what network codes ask of it and what the hardware is tuned for. So in ordinary operation an 800 VA microinverter is producing something very near 800 W, and the distinction looks academic.
It stops looking academic the moment a device is configured otherwise. Many microinverters have an adjustable power factor, because distribution networks sometimes want generation to absorb or supply reactive power to help hold voltage steady. Turn that dial and the same hardware that’s declared at 800 VA is delivering meaningfully less than 800 W.
Run the arithmetic the other way and you get the case that actually bites. A device declared at 800 W of real output, operating at a power factor of 0.9, is putting out 889 VA. That’s inside the statutory definition’s 800 watts and outside the specification’s 800 VA. A kit sold on an "800W" badge isn’t automatically a kit that meets the specification.
One manufacturer, two units, one document
This isn’t hypothetical. Hoymiles’ own European user manual for the HMS-600W/700W/800W/900W/1000W-2T series (Region: Europe, V202405) does both things inside one document.
The features list in the product description reads "Maximum output power up to 600/700/800/900/1000 W". The technical data table at the back has a row headed "Rated output power (VA)", and the value in the HMS-800W-2T column is 800. Same document, same device, watts in the marketing line and volt-amps in the spec table. The model name itself carries a W.
The same table lists an "Adjustable power factor (@nominal power)" of ">0.99 default", adjustable across "0.8 leading … 0.8 lagging". At the default that’s just over 792 W of real output. At the bottom of its adjustment range, 800 VA is 640 W. Our full read of the HMS-800W-2T datasheet goes through the rest of the numbers, and the microinverter comparison covers how the main brands present their ratings against each other.
None of this makes Hoymiles unusual or careless. The industry writes it this way, which is exactly why a UK buyer can’t treat a W on a box as evidence of anything about VA.
So which number binds a UK kit?
Both of them, as the documents currently stand. A device has to fall inside the statutory definition to use the plug-in route at all, and it has to comply with the specification the SI names.
Read strictly, the two limits point the same direction. Apparent power is always greater than or equal to real power, so any inverter that genuinely stays under 800 VA can’t exceed 800 watts. On that reading the specification is the tighter of the two and satisfying it satisfies both.
That arithmetic doesn’t settle the drafting, though. It doesn’t tell you which figure a manufacturer is obliged to declare, which one is measured in a dispute, or how a device rated at 800 W real output with a power factor of 0.9 would be treated. We checked all three published documents for an answer. SI 2026 No. 848 never uses the words "apparent power", "volt-amp", "reactive" or "power factor". The specification never uses "watt". The government response published alongside it writes the 800W limit that way eight times, uses "800VA" once when summarising what respondents proposed, and mentions power factor and apparent power nowhere at all. So the mismatch isn’t explained, resolved or acknowledged in any of them.
Our honest position is that this needs clarifying by DESNZ or by the BSI standard that eventually replaces the interim specification, and until it is, buyers should size to the tighter figure. The compliance checker works to the 800 VA number for that reason.
Why 3.5 A does more work than either figure
The 3.5 A limit in clause 4.1 gets far less attention than the headline number, and on a real solar setup it’s frequently the binding constraint.
At the UK’s nominal 230 V, 800 VA works out at 3.48 A. That’s why the two limits look like one limit: they’re the same constraint expressed twice at nominal voltage. Hoymiles’ table shows the 800 VA unit at 3.48 A on a 230 V supply, and it’s no coincidence.
Mains voltage doesn’t sit at 230 V, though. Drop the supply and the same apparent power needs more current. Hoymiles’ own table gives 3.64 A for the 800 VA model on a 220 V nominal supply. The UK’s nominal is 230 V, so that column is for other markets, but it shows the direction of travel, and 3.64 A is already above the specification’s 3.5 A ceiling. The specification anticipates this: its type-test table requires the current to be measured "at reduced voltage (≈0.85 Un)" using the procedure in BS EN 50438 and G98, with the pass criterion ≤ 3.5 A. A compliant inverter has to hold current below 3.5 A even when the voltage sags, which means backing off its apparent power output to do it.
That’s also the reason the specification gives for the one-device rule, phrased as keeping "the currents flowing through the electrical connection to the consumer unit within safe levels". The specification allows one device per final ring circuit, but its own note records that G98 Issue 2 Amendment 1 2026 restricts it to one device per household "unless and until that is amended". One per household is the limit that actually applies, and we’ve unpicked the per-circuit rule and why it misleads separately.
The DC side has four separate limits, not one
The AC figure is only half the sizing question. On the panel side, the specification sets four constraints that a kit has to satisfy independently.
| Limit | Value | Where it sits |
|---|---|---|
| Total PV module DC power | 2,000 W | Clause 4.1, retained from v1 and flagged as under review |
| PV modules per inverter | 4 | Clause 4.2.2 |
| PV modules in series per string | 2 | Clause 4.2.2 |
| Open-circuit voltage at the inverter inputs | 120 V DC | Clause 4.2.2 |
These stack rather than substitute. A solar panel kit can sit comfortably inside the 2,000 W DC cap and still breach the module count, the series limit or the voltage ceiling, so each one needs checking on its own. Together they answer the question of how many solar panels you can hang off one device: four, at most, and never more than two of them in series. The specification’s stated reason for the 120 V figure is arc risk, since "the risk of sustained DC arcing increases with DC voltage", so the ceiling exists "to reduce the likelihood and consequences of DC arc faults".
Fitting more panel wattage than the inverter can export is normal practice rather than a fault. Two 400W panels give 800 W of DC panel capacity, and on a typical UK balcony, near vertical and rarely south-facing, that array will spend most of the year producing a fraction of it. An oversized array recovers system output in dull weather, in winter, and where shade takes out part of it, which is also why a microinverter with two independent MPPT channels behaves better on a partly shaded railing than one that tracks everything together.
What extra panel wattage can’t do is raise the AC ceiling. Inverter capacity is fixed at 800 VA, any excess power above that is clipped and simply not harvested, and the route itself doesn’t permit a larger inverter. Note too that the DC cap is measured against the modules’ rated maximum power, so it’s the number on the panel label that counts rather than the solar power the array actually makes.
960 W is advice, not a limit
This one gets misreported constantly, so it’s worth being exact.
Clause 4.1 says manufacturers "shall advise that consumers installing plug-in solar devices with a total PV module maximum power above 960W should consider professional assessment of their existing electrical installation prior to installation". The specification’s note calls 960 W "a precautionary measure for larger plug-in solar", drawn from the same principle in the German DIN VDE 0126-95.
What crossing 960 W does and doesn’t mean
What’s mandatory is the advice, not the assessment. A kit with more than 960 W of panels is fully compliant. The manufacturer has to tell you to think about getting your wiring looked at, and you’re free to conclude it’s fine. The hard DC cap is 2,000 W. Crossing 960 W isn’t a compliance failure and doesn’t require an electrician.
That said, the advice isn’t empty. The specification also tells manufacturers to say that where an installation "is using older fuse protection and does not incorporate RCBOs, the installation shall be checked and, where necessary, upgraded by a professional electrician". If your consumer unit is old, that’s the sentence worth acting on.
What to check on a datasheet before you buy
Sizing runs backwards on this route. The rules fix what size inverter you may use, and you pick solar panels around it. Most plug-in solar kits publish enough to check that, and reading a spec sheet with the units in mind gets you most of the way.
- Find a declared apparent power in VA. That’s the figure the specification’s type test measures. A listing for an "800W inverter" tells you nothing on its own, and neither does a model name.
- Check the rated output current against 3.5 A, and check what it becomes at the bottom of the voltage range rather than only at 230 V.
- Check the power factor row. A wide adjustable range is normal, and it’s the reason the W figure and the VA figure can drift apart.
- Add up the solar panel labels against 2,000 W, then count modules, count series strings, and add the open-circuit voltages against 120 V.
- Check the ENA Type Test Register listing and the on-product declaration. A CE or UKCA mark isn’t the compliance test here. The register records its plug-in solar entries in kilowatts, a third unit again, listing them at 0.8 kW; as at 11 August 2026 all four entries under that device type are assessed Non-compliant, so none of them clears that gate yet.
Our step-by-step on checking a kit against all three gates covers the register search and the declaration in more detail. Two things the units check won’t tell you: a battery system inside the device puts it outside the statutory definition altogether, and whatever the numbers say, notifying your DNO is mandatory under the specification, which requires the product itself to carry that statement and a QR code to the guidance.
Frequently asked questions
What’s the difference between 800 VA and 800 W in a solar inverter? VA is apparent power, the voltage multiplied by the current, and it’s what the cable and fuse have to carry. W is real power, the part that does useful work and appears on your bill. They’re related by the power factor: W = VA × power factor. At a power factor of 1 they’re identical, and at 0.9 an 800 VA inverter delivers 720 W.
Is there a plug-in solar inverter rated at 800 W? Plenty of microinverters carry 800W in the model name or the marketing line. Look at the specification table and you’ll usually find the rated output declared in VA instead. The UK specification measures apparent power, so the VA figure is the one to compare against the limit.
Can I connect more than 800 W of solar panels to an 800 VA inverter? Yes, up to 2,000 W of rated module power, and it’s normal practice. You also have to stay inside four PV modules, no more than two in series in any string, and 120 V DC open circuit at the inverter inputs. Above 960 W of panels the manufacturer must advise you to consider a professional assessment.
What does inverter clipping mean? Clipping is what happens when the panels can produce more than the inverter can export. System output flattens at the ceiling and the surplus is never harvested. Solar never delivers constant power anyway, so on a UK balcony an 800W plug-in device clips for fewer hours than the panel rating suggests, because near-vertical panels rarely reach their rated output.
Is an 800 VA kit worth it in the UK? The government’s own estimate for annual bill savings is £70 to £110, and a balcony sits at the low end of that, because the upper figure models a 30 degree south-facing installation rather than a vertical railing. Our savings guide sets out the assumptions. Battery storage would change the self-consumption maths, but battery-integrated devices can’t use this route.
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