How many solar panels do I need?

Between eight and sixteen for most UK homes, but the number is an output rather than an input. It falls out of two figures you already have: how much electricity you use, and how much usable roof you own.

Anyone who answers with a panel count before asking for either is quoting a default, not sizing a system.

Start with your consumption

Find your annual electricity use in kilowatt hours. It is on your annual statement and in your supplier’s app.

That number sets the ceiling on what solar can save you, because the value comes mainly from not importing electricity you would otherwise have bought: 26.11p per kilowatt hour avoided at the current capped rate, against an export rate of around 12p at best for surplus. Generating far beyond your consumption means selling the excess at less than half what you pay to buy.

Turn it into capacity. Our model assumes 850 kilowatt hours per year per kilowatt of south facing panels, so a household using 3,600 kilowatt hours is fully matched, on an annual basis, by about 4.2 kilowatts. With current residential panels around 430 to 540 watts each, LONGi’s Hi-MO X6 datasheet being a representative example, that is roughly nine or ten panels.

So the first question is not how many panels fit. It is how many you can usefully consume, and for most households the answer starts near ten.

Then measure the roof

What you can fit is a physical question:

1. Identify the pitches facing somewhere between east and west through south. North-facing pitches rarely earn their keep.
2. Measure the usable area, excluding chimneys, vents, roof windows and the margins installers must leave at the edges.
3. Divide by the area of a modern panel, roughly two to two and a half square metres for the current large formats, to get a panel count.
4. Multiply by the panel wattage on the quote to get capacity in kilowatts peak.

A worked example: a rear pitch six metres wide with a four metre slope is 24 square metres; edge margins and a roof window leave perhaps 19, which is eight of the larger panels, about 4.3 kilowatts. A satellite view and a tape measure get you close enough to sanity check any quote you are given, and an installer’s survey refines it.

Where the two answers meet

You now have two numbers: what your roof can hold, and what your household can absorb. One of them is your binding constraint, and which one it is changes the right decision entirely.

If the roof is the constraint, higher efficiency panels are worth paying for, because they fit more capacity into the same area. This is common on terraces and on properties with one small usable pitch.

If consumption is the constraint, higher efficiency panels are wasted money. The better spend is on raising how much of your generation you use: shifting loads into daylight, a diverter for hot water, or storage, in that order of cost effectiveness.

What raises the number you can absorb

Several things increase household consumption enough to justify more panels:

• An electric car charged at home, worth two or three thousand kilowatt hours a year at typical mileages.
• Electric heating, or a heat pump.
• Someone at home during the working day.
• A battery, which lets evening consumption draw on midday generation.

If any of these is coming within a couple of years, size for the household you will be rather than the one you are. Adding panels later means paying for scaffolding and a site visit a second time.

Why the marginal panel is cheap

A large share of an installation’s cost does not scale with size: scaffolding is priced by the property, the survey, certification and network notification are fixed, and one inverter covers a range of array sizes, as the full cost breakdown shows.

The government’s own installation cost data confirms the effect at national scale: systems in the 4 to 10kW band had a 2025/26 median of £1,697 per kilowatt against £1,780 for the sub-4kW band. So going from ten panels to fourteen costs far less than forty per cent more, and the cost per kilowatt peak falls as the system grows. This is the strongest argument against reflexively accepting the size you were first quoted, and against the bedroom-count tables that stamp one default on every three bed house.

The limit is not the roof running out. It is your consumption running out, at which point extra generation earns the export rate rather than saving the import rate, and the marginal panel’s return halves.

A worked sequence

1. Annual consumption from your bill.
2. Usable roof capacity from the measurement above.
3. Estimated annual generation for that capacity, from a location specific tool, the European Commission’s PVGIS is free, rather than a national average.
4. Your realistic daytime share, from half hourly smart meter data if you have it.
5. Compare the generation you would actually use against the generation the roof could produce.

Where those two diverge sharply, you have found your constraint, and with it the decision that actually matters.

Then check it against a model

Panel count is a proxy for the thing you care about, which is what the system saves. Put your consumption, your rates and a candidate system size into the ROI calculator, which treats import and export separately, and try two or three sizes: the step from ten panels to twelve, or twelve to fourteen, shows up directly in the payback year. The right number is usually obvious once you can see what each one returns, and it is rarely the number on the first quote.