400 W vs 800 W plug-in solar: which size should you buy?

What do “400 W” and “800 W” actually mean?

The legal limit is 800 W of AC output from the inverter (SI 2026/848). Panels can add up to 2,000 W (IPS §4.1).

Retailers use “400 W” and “800 W” loosely: sometimes for the inverter, sometimes for the panels. What decides output is the panel total and, above about 1.2 kW of panels, the 800 W cap.

On the ENA register, most Compliant plug-in kits are registered at 0.8 kW: 73 of the 84 Compliant entries on 29 September 2026. All but one of the other 11 are 0.5 kW or less. Check the kit’s entry on its kit page or the ENA register.

In this guide, “one panel” means about 440 W of panel and “two panels” about 880 W.

One kit per household

The network rules allow only one plug-in solar device per household (Ofgem, approving the G98 change). You can’t start with one panel and add a second kit later.

Does two panels mean double the output?

Close to it. Output scales with panel area. From PVGIS (2005–2023 averages, south at 35° unless stated):

City	One panel (0.44 kWp)	Two panels (0.88 kWp)
London	448 kWh	897 kWh
Manchester	390 kWh	780 kWh
London, vertical	327 kWh	654 kWh

At 0.88 kWp the 800 W cap causes no measurable clipping. See how much electricity.

Does two panels mean double the saving?

No. You save money only on electricity you use as it’s made. The rest goes to the grid, usually unpaid (see export).

We worked this out from a PVGIS hourly series for Bristol, south at 35°, for 2020 (943 kWh for two panels, 472 kWh for one). We matched each hour’s output against a steady background load: the fridge, freezer, router and anything else that runs all day.

Electricity used at home

Steady background load	One panel	Two panels	Extra from the second panel
100 W	263 kWh (56%)	315 kWh (33%)	53 kWh
150 W	340 kWh (72%)	429 kWh (45%)	89 kWh
200 W	397 kWh (84%)	525 kWh (56%)	128 kWh

Yearly saving at 26.32p/kWh

At the October–December 2026 GB average cap (Ofgem):

Steady background load	One panel	Two panels	Second panel adds
100 W	£69	£83	£14
150 W	£89	£113	£23
200 W	£104	£138	£34

Limits of this working

2020 was a sunnier year than average at this spot. Hour-by-hour matching also slightly overstates how much you use yourself, because short spikes and dips even out within the hour. The relative picture, with the second panel worth less than the first, holds either way.

Which size suits you?

Two panels (around 800 W) if	One panel (around 400 W) if
Someone’s home in the day, or you run appliances in daylight	The house is empty most of the day
Background load is 150 W or more	Background load is low
You have space for two panels in sun	You only have space for one, such as a small balcony
Daytime electricity is expensive on your tariff (see best tariffs)	You want the lowest cost per kWh actually used, rather than the most kWh

A smart meter’s overnight reading is a good guide to your background load.

On cost, compare price per kWh you’ll use, not price per watt. Divide the price by ten years of self-used output from the calculator. The method is in cost.

What about more than 800 W of panels?

Kits can carry up to 2,000 W of panels, but the 800 W inverter cap clips the peak. In our Bristol working, going from 0.88 to 2.0 kWp adds around 80% more output, not 127%. Most of the gain comes in winter and at the start and end of the day.

Above 960 W of panels, manufacturers must advise you to “consider professional assessment” of your wiring (IPS §4.1).

What to do next

Try both sizes in the calculator with your own usage.
Filter single-panel and 800 W kits on in stock, or answer a few questions in the kit finder.