800W Plug-In Solar vs 400W: Which Size Is Right in the UK

In the UK, “800W” on a plug-in solar kit is the inverter’s maximum output, not the panel size; up to 2,000 W of panels may sit behind it. A “400W” kit is usually one 400 to 500 W panel. Two panels roughly double the electricity produced and, for a household at home in the day, add about £45 to £50 a year of savings for an extra £100 to £300.

Inverter watts and panel watts are different things

A plug-in kit has two ratings, and listings often blur them.

• Inverter output is what flows into your socket. SI 2026/848 limits a plug-in microgenerator to a rated AC output of 800 W, and the Interim Product Specification sets 800 VA and 3.5 A. Every compliant UK kit sits at or below this.
• Panel capacity is the sum of the panels’ laboratory ratings, measured under bright sunshine at 25°C. The specification allows up to 2,000 W in total, from up to four panels, with no more than 120 V DC at the inverter. Above 960 W, manufacturers must advise you to consider a professional check of your electrical installation.

So a kit sold as “800W” may be a single 465 W panel with an 800 W microinverter, or two 450 W panels with the same inverter. Compliant kits listed in October 2026 ranged from one 460 W panel to about 1,260 W of panels. British daylight rarely reaches laboratory conditions, so a 450 W panel spends most of the year producing a fraction of its rating, and an 800 W inverter fed by 900 W of panels hits its ceiling only on the brightest hours of the year.

Output: one panel against two

We used PVGIS, the European Commission’s solar model, to estimate yearly output for London with standard 14 percent system losses, using hourly data for 2019 to 2023 and capping the output at 800 W to mimic the inverter. These are modelled estimates, not measurements, and hourly averages slightly understate clipping on bright days.

Table scrolls sideways

Up to about 1,000 W of panels, output rises almost in step with capacity: the inverter limit costs next to nothing, because in the UK the panels rarely produce their full rating. Beyond that the extra panels still help, but with diminishing returns: at 2,000 W, about a sixth of the potential output is lost at the 800 W ceiling in a south-facing, tilted position, and less on a vertical east or west railing.

The government’s own model, which uses average UK sunshine rather than London’s, is more conservative: 690 kWh a year for 800 W tilted south and 378 kWh vertical east or west. PVGIS gives about 700 kWh for the tilted case in Manchester and about 714 kWh in Edinburgh, so treat the London figures as the upper middle of the British range.

Savings: why one panel loses less than half

A plug-in kit only saves money on electricity you use at the moment it is produced. The government’s model shows the effect: its larger case uses 62 percent of the output at home, its smaller case 72 percent.

Applying that pattern to one and two panels gives an illustration for a household at home during the day, at the October to December 2026 price cap of 26.32p per kWh. These are our assumptions, scaled from the government’s model, not measurements:

Table scrolls sideways

The second panel doubles generation but adds about two-thirds to the saving, because more of its output arrives at midday when the home is already supplied. If you are out all day, the gap narrows further; if you shift washing and dishwashing into daylight, it widens. Our guide to how much plug-in solar saves explains the drivers.

Winter and the dull days

The case for more panel capacity is strongest in the darker half of the year. In the same London model, November to February output for a 30-degree south-facing kit was:

Table scrolls sideways

In winter almost every unit is used at home, because output rarely exceeds what a fridge, router and standby devices draw. So extra winter generation turns almost fully into savings. See plug-in solar in winter for the monthly picture.

Space, weight and the mount

Panel capacity is limited less by the rules than by your balcony. A typical glass panel of around 450 W measures roughly 1.7 by 1.1 metres and weighs about 20 to 25 kg. Two of them need a railing or wall that can carry around 50 kg plus wind load, and a balcony long enough for two panels side by side.

The kit’s mounting system must come with a structural analysis for UK wind and snow loads and maximum installation heights, so check that two panels are permitted at your height and on your type of railing. In England, permitted development limits how far panels may project from a wall or balcony; see planning permission for plug-in solar.

Which size suits you

Table scrolls sideways

Choosing between two listings

For the wider buying criteria, see how to choose a plug-in solar kit in the UK; for what the cheapest kits include, £400 plug-in solar kits.

Next steps

• The best direction and angle for plug-in solar in the UK
• How plug-in solar panels work
• Where to buy plug-in solar panels in the UK