Most UK homes that go solar end up with somewhere between 8 and 14 panels, which is a system of roughly 3 to 5 kilowatts. That is the honest ballpark, but it is not an answer, because the right number for your house depends on two things a salesperson often skips over: how much electricity you actually use, and how much of the roof you can point somewhere near south. Get those two straight and you can size the array yourself, spot when a quote is pushing more panels than you will ever use, and understand why the biggest system is rarely the best value.
The short answer. Size the system to your electricity use, not to the roof. A typical home using around 3,500 kWh a year is well served by about 4 kWp, which is roughly 9 or 10 modern panels and needs about 20 square metres of unshaded roof. Each kilowatt of panels makes around 850 to 950 kWh a year on a decent south-facing British roof. Going much bigger only pays if you can use or store the extra, because what you export earns far less than what you avoid buying.
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Start with your electricity use, not the roof
The question people ask is "how many panels fit on my roof", when the one that matters is "how much electricity do I use, and when". A solar panel only saves you money on the units you would otherwise have bought from the grid at the full rate, currently about 26.32p per kWh under the Ofgem cap for October to December 2026. Any generation beyond what the house can use at that moment gets exported, and export pays a good deal less. So the sensible target is a system that roughly covers your daytime demand across the year, not one that maxes out every tile.
Find your annual use first. It is printed on your bill, or add up a year of meter readings, or read it off your smart meter display. A small flat might get through 1,800 kWh a year, a typical three-bed house around 2,700 to 3,500 kWh, and a larger family home with an electric car or a heat pump can be well past 5,000. If you are not sure where you sit, the typical home electricity use guide has the usual figures. That one number is the foundation for everything below.
How much a single panel makes in a British year
Panels are rated in watts of peak output, written Wp, measured under bright test conditions. A modern domestic panel is commonly 400 to 450Wp, so ten of them give a 4 to 4.5kWp array. The rating is not what you get day to day, though; it is a label for comparing panels, like the engine size of a car rather than its real fuel economy.
What a system actually generates over a year is better captured by a rule of thumb: in the UK a well-sited array makes somewhere around 850 to 950 kWh for each kWp installed. A south-facing roof at a sensible pitch with nothing shading it sits near the top of that; an east or west roof, or one that a chimney or tree shades for part of the day, sits lower. Scotland and the far north come in a touch below the south coast, but the gap is smaller than most people expect. The direction and angle guide explains how much orientation really costs you, and the solar output calculator lets you put your own roof in and see a figure.
| System size | Panels (at ~430Wp) | Yearly output (good roof) | Roof space needed |
|---|---|---|---|
| 2 kWp | about 5 | ~1,800 kWh | ~10 m² |
| 3 kWp | about 7 | ~2,700 kWh | ~15 m² |
| 4 kWp | about 9 to 10 | ~3,600 kWh | ~20 m² |
| 5 kWp | about 12 | ~4,500 kWh | ~25 m² |
| 6 kWp | about 14 | ~5,400 kWh | ~30 m² |
Illustrative figures using about 900 kWh per kWp a year for a decent south-facing roof. An east or west roof, or any shading, lowers the output. Your own numbers depend on the panels, the roof and the shade.
A worked example: sizing a typical house
Take a three-bed house that uses 3,500 kWh of electricity a year, with a south-west facing roof and no significant shade. Match the system to the use and you land on about 4 kWp, which at 430Wp panels is nine or ten of them and needs roughly 20 square metres of clear roof. On that roof a 4kWp array might generate about 3,600 kWh across the year, so on paper it nearly matches the home's total use.
The catch is timing. Solar makes most of its power in the middle of the day, in summer, and a lot of a home's demand lands in the dark mornings and evenings, especially in winter. Without a battery, a house typically uses only something like 30 to 50 per cent of what the panels make at the moment they make it; the rest is exported. So from that 3,600 kWh, perhaps 1,400 to 1,800 kWh is used directly, saving around £370 to £470 a year at 26.32p. The exported remainder, say 1,900 kWh, earns a much smaller export rate, maybe around £285 at an illustrative 15p. The lesson is plain: the first few kilowatts of panels save you the most, because they displace the units you were always going to use, and each extra kilowatt beyond your daytime demand earns steadily less.
Will they actually fit?
Roof space is the second limit, and it is simple arithmetic. A single panel is roughly 1.7 metres by 1.1, a little under two square metres, so as a working figure allow about 5 square metres per kilowatt of panels once you leave room at the edges. A 4kWp system therefore wants around 20 square metres of usable roof, which most three-bed semis have on one pitch. Measure the clear area, knock off anything taken up by a chimney, a vent, a roof window or a valley, and see what is left.
If one roof face is too small, panels can be split across two faces, for instance an east slope and a west slope, which actually spreads generation more evenly across the day even though the peak is lower. Flat roofs work too, with the panels on angled frames, though they must be spaced so they do not shade each other. The thing to avoid is squeezing panels onto a face that a tree or neighbouring building shades for much of the day, because shade on even one panel can drag down a whole string.
Why bigger is not automatically better
It is tempting to fill the roof, and installers will happily quote for it, but past the point where the panels match your daytime use the economics tail off. Every extra kilowatt still costs much the same to buy and fit, yet the power it makes increasingly gets exported rather than used, and export is worth far less than the grid price you avoid. A panel that saves you 26.32p a unit is doing four times the work of one whose output you sell for around 15p.
There are good reasons to go a little larger than today's use. If you are about to get an electric car, add a heat pump, or know your household is growing, future demand can justify more panels now while the scaffolding is up. But sizing far beyond any plausible use, purely because the roof is big, is how people end up disappointed with the payback. The is solar worth it guide walks through the full sum.
Where a battery changes the maths
A home battery does not make the panels generate more; it lets you use more of what they already make. Store the midday surplus and draw on it in the evening, and your self-consumption can climb from that 30 to 50 per cent up towards 70 or 80. That turns exported units worth 15p into avoided units worth 26.32p, which is where the extra value comes from, and it can make a slightly larger array worth having because you are no longer giving the surplus away. Whether the battery itself pays for itself is a separate question, covered in the solar battery storage guide, but it clearly shifts the sizing logic: with storage, a few more panels start to earn their keep.
What I would actually do
I would ignore the roof at first and write down one figure, my annual kWh, then pick a system that roughly matches my daytime share of it rather than my total. For most homes with no battery that means something around 3 to 4 kWp, which comfortably fits a normal roof and puts the money where it works hardest. If a battery is in the plan, or an EV or heat pump is coming, I would stretch to 4 to 5 kWp and no further without a clear reason. Then I would get two or three quotes and treat any that pushes a much larger array, without asking a single question about how and when I use electricity, as a quote to walk away from. Size it to your life, check it against the payback calculator, and the number of panels sorts itself out.