How to Size an 800W Plug-in Solar Kit in the UK

Published 17 September 2026

A UK plug-in solar kit is capped at 800 VA on the inverter. That is a legal ceiling, not a target, and it is why every balcony kit is sold as "800W". Export is unpaid, so a kit larger than the load you run while the sun is up gives its extra kilowatt-hours away. Occupancy, not the cap, is usually what sizes the kit. The rules themselves are on the plug-in solar rules page; this page is only how big the kit should be.

Two limits, and they are not the same

The inverter cap is 800 VA of AC output. Panel wattage may be higher: 880 W or 1,000 W behind an 800 VA microinverter is normal, because British irradiance rarely hits the STC rating and the extra DC earns its keep on grey mornings. That is clipping, and in UK light it costs about 1% at 880 W and about 2.5% at 1,000 W on the plug-in solar savings calculator. It is not a reason to buy the biggest kit on the page.

The economic cap is the load that is on while the kit is generating. There is no export tariff on this route. Every watt the kit makes above what the flat is using at that moment is spilled to the grid unpaid. A bigger kit does not store the surplus. It does not shift it to the evening. It just makes more unpaid electricity at noon.

If you need the legal tests (G98 evidence, no extension lead, no battery in the box, timber balcony and cladding), use the plug-in solar rules guide. If you need rail brackets and consent, use balcony solar UK. If you need who is selling, use best plug-in kits. This page assumes a legal kit and asks how big it should be.

Size to daytime coincident load

Walk round the flat at 11am on a weekday and add up what is actually on: fridge, router, boiler standby, a laptop, maybe a fish tank. For a lot of working households that total is 150 to 300 W. That is the number the kit should be able to cover, not 800 W, and not the annual kWh on the bill.

The annual bill is the wrong input. Most of a household's kilowatt-hours happen in the evening and in winter. A plug-in kit only offsets what it can touch in daylight. Using 3,000 kWh a year as the sizing target will always recommend the 800 VA cap, and will always overstate the saving.

• Empty on weekdays, home at weekends. Daytime coincident load maybe 150 W. A 400 to 500 W kit is the honest size. An 880 W kit will spill most of Monday to Friday.
• Working from home, lights and a PC. Coincident load 200 to 400 W. A 600 to 880 W kit starts to earn its extra panels.
• Home, and you will put the washing machine and dishwasher on a midday timer. Coincident load can exceed 800 W whenever those run. The inverter cap is now the limit. Buy the 800 VA kit, oversize the panels a little, and actually use the timer. The timer is worth more than the last 200 W of panel.
• Garden or tilted frame, south, someone in during the day. Yield is about 40% higher than a vertical rail (the savings calculator uses 1.00 versus 0.72). The same occupancy then supports a larger kit before spill dominates.

Worked examples: Midlands, vertical south, 25.7p, £499 vs £299

Same balcony as the balcony guide: Midlands specific yield 1,010 kWh/kWp, vertical south 0.72, unshaded. Two kits, three occupancy rates. Occupancy here is the share of annual generation used in the home, which is a rough proxy for being in and running loads while the kit generates. Export is worth £0.00.

Payback, ignoring tariff inflation: the 400 W kit at 25% occupancy is 16 years, which is not a kit you should buy. The 880 W kit at 25% occupancy is 12 years, which is also not a kit you should buy. At 60% occupancy the 880 W kit is about 5 years and the 400 W kit is about 7 years, so the extra panels now earn their keep. At 80% occupancy the 880 W kit is under 4 years.

The lesson is not "always buy 880 W". It is that occupancy moves the answer more than panel wattage does, and that nobody at 25% self-use should be shopping at the 800 VA cap. If the 25% row looks like your week, either run wet appliances on a timer (which is how you climb toward 60%) or spend the money on a tilt frame rather than on more panel. Tilt is in the balcony guide; it is the one hardware change that raises yield without raising spill in the same proportion, because it makes more in winter mornings when you are actually home.

Put your own balcony through the savings calculator and the UK solar map before you pick a wattage. The map is the yield; the calculator is the unpaid spill.

How to measure coincident load without a fancy meter

You do not need a clamp meter. A cheap plug-in energy monitor on the fridge, a look at the smart-meter app between 10am and 2pm, and a list of what is always on will get you within 50 W, which is close enough to choose between a 400 W kit and an 880 W kit.

• Always-on. Fridge (typically 20 to 40 W average, 100 W when the compressor runs), router, boiler standby, smoke alarms, a fish tank if you have one. Call it 80 to 150 W for a small flat.
• Work-from-home stack. Laptop plus screen plus a lamp is another 80 to 150 W, but only on the days you are actually there.
• Wet appliances. A dishwasher or washing machine is 1,200 to 2,000 W while the heater is on, then much less. One timed cycle in the midday window will swallow an entire 800 VA kit for an hour. That is the occupancy cheat: you did not raise the always-on load, you moved a load you were going to run anyway.

If the smart-meter app shows 200 W at 11am on a workday and 1,800 W at 7pm, believe the 200 W for this kit. The 1,800 W is not a plug-in-solar problem. Oversizing the kit to "cover the evening" does nothing, because the kit is dark by then and has no battery.

Clipping is not a way around 800 VA

Sellers will offer 880 W or 1,000 W of panel on an 800 VA inverter and call it oversizing. In the savings calculator the clip loss is about 1% at 880 W and about 2.5% at 1,000 W in UK light. That extra DC is useful at 9am in November. It is not extra AC at noon in June. If your occupancy cannot use 800 VA, 1,000 W of panel just means more unpaid spill on the days the inverter is already at the cap.

When to go all the way to 800 VA

• You are in during the day, or you will automate the washing machine, dishwasher or immersion onto a midday window.
• The mounting is tilted, not a vertical rail, so winter production is worth having.
• The socket is a real fixed 13 A outlet, not a plan to run a reel, and the kit is a complete solar-only product.
• You have already accepted that 1,000 W of panel behind 800 VA is a winter play, not a way around the cap. The inverter still stops at 800 VA on a clear June noon.

Do not add a battery to stay inside this sizing. A battery in the box takes the kit outside the socket route and back to an electrician. If you want storage, that is a different system and a different page: how to size an off-grid battery bank.

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