Most UK homes land on 8–14 panels, a 3–5 kWp system. Divide your annual electricity use by local yield, then check the roof fits. 3 to 6 kW compared.
Most UK homes need somewhere between 8 and 14 solar panels, which with today's 400–450 W panels means a system of roughly 3–5 kWp. A one or two-person household with modest bills sits at the bottom of that range; a family home with an electric car or heat pump on the horizon sits at the top, and often goes past it to 6 kWp. The number that decides where you land is how much electricity your home uses in a year. The roof then either accommodates that or forces a compromise.
Here is the method in three steps you can do from your energy bill, then the 3, 4, 5 and 6 kW systems installers quote most often, side by side. A firm figure is a survey job, and we say so.
Start with the kWh you use per year, from your annual statement or energy app. If you cannot find it, Ofgem's typical domestic consumption values are a benchmark: a medium household uses around 2,700 kWh a year, a high-use household around 4,100 kWh, though those values are revised from time to time. Then add anything you know is coming: an EV typically adds 2,000–4,000 kWh a year, and a heat pump commonly adds 2,500–4,500 kWh in place of the gas it replaces.
Now divide by what one kWp of panels makes. The modelled rule of thumb is that every 1 kWp generates roughly 850–1,000 kWh a year on an unshaded, roughly south-facing UK roof. Real roofs do less: across 14,879 metered UK homes through 2024, the middle half made 677–838 kWh per kWp, and postcode-area medians run 677–919, because real roofs face every direction, collect shade and age. We set out how those figures are produced at how we measure.
Worked through: a home using 3,500 kWh a year divides by roughly 950 and lands on about 3.7 kWp, so 9 or 10 panels, if the roof is a good one. On a typical real roof the same home needs closer to 4.5 kWp, and which of those you are is what a roof check settles. Per-panel output is covered in how much electricity does a solar panel produce?.
The sum tells you what you would like. The roof tells you what you can have. A modern panel is roughly 1.7–2 m², and once you add the margins installers leave at ridges, eaves and verges, budget roughly 2–2.2 m² of clear roof per panel. Chimneys, rooflights and dormers eat into that, and only pitches facing roughly south, east or west count.
Output below is for a good, unshaded roof. Prices are 2026 installed figures for panels and inverter, before a battery, and quotes vary by roof, region and installer.
| System | Panels (400–450 W) | Usable roof needed | Annual output (good UK roof) | Typical installed price | Who it suits |
|---|---|---|---|---|---|
| 3 kWp | 7–8 | roughly 15–18 m² | roughly 2,550–3,000 kWh | typically £5,000–£6,500 | One or two people, modest bills, or a small roof |
| 4 kWp | 9–10 | roughly 19–23 m² | roughly 3,400–4,000 kWh | typically £6,000–£7,500 | A typical three-bed on gas heating, no EV yet |
| 5 kWp | 11–13 | roughly 23–29 m² | roughly 4,250–5,000 kWh | typically £7,000–£9,000 | Family homes, higher use, or planning a battery |
| 6 kWp | 13–15 | roughly 27–33 m² | roughly 5,100–6,000 kWh | typically £8,000–£10,500 | EV or heat pump households with the roof for it |
A battery adds roughly £2,500–£5,500, and residential solar and batteries are zero-rated for VAT until 31 March 2027. What moves a quote within those ranges is covered in how much do solar panels cost in the UK?.
Ten years ago the sensible advice was to size conservatively. Four things have changed:
Adding panels later means scaffolding again, so if any of the four apply within the system's life, size for them now.
You will hear "3.68 kW" a lot. It is the export limit, per phase, under G98, the route where your installer fits the system and simply informs your network operator afterwards. Almost every UK home is single-phase, so 3.68 kW of inverter output is the largest system you can connect without asking first.
Above that, the system falls under G99: in plain terms, the installer applies to the network operator before installation and waits for approval. In most cases that takes a few weeks and is granted, sometimes with an export limit attached. Your installer handles the application; your job is to build the time in.
The limit applies to the inverter's output, not the panels' rating, so installers routinely fit 4.5–5 kWp of panels behind a 3.68 kW inverter. UK panels rarely hit full rating, so little is lost to the cap over a year. If the array you want is comfortably above 5 kWp, expect G99 and treat it as paperwork rather than a barrier.
A tempting shortcut is to match annual output to annual usage and call it done. The trouble is timing. Panels generate in the middle of the day and mostly between April and September; homes use electricity in the evening and mostly in winter. So a 4 kWp system making 3,800 kWh on a home using 3,800 kWh does not zero the bill.
Without a battery, a typical UK household directly uses around 30–50% of its own generation. The rest is exported under the Smart Export Guarantee at a rate set by your supplier, which varies widely, changes often, and is usually well below the import price. With a battery, self-consumption commonly rises to 60–80%. So without a battery, going far past your annual usage buys exported kWh at a low rate and stretches the payback on the last few panels; with a battery or an EV, the extra panels have somewhere to go.
The good-roof yield assumes a roughly south-facing, unshaded pitch. East and west pitches produce noticeably less per panel, although a split east-west array spreads generation across the day in a way that suits homes without a battery. We rank the options at which roof direction is right for solar panels?.
The sizing effect is simple: a roof that yields less per panel needs more panels for the same output. An east-west home wanting 4,000 kWh a year might need 5 kWp where a south-facing neighbour needs 4, and shading from a chimney, tree or extension works the same way. This is where a generic table stops helping. Our roof check looks at your roof from the address, estimates its usable area and direction, and returns a panel-count and kWp range for that roof rather than for an average house.
Everything above gets you to a sensible bracket. A survey turns it into a design: a surveyor measures the pitches rather than estimating them, checks the roof structure can carry the load, maps shading across the year, confirms your supply is single or three-phase, and decides on string layout and inverter size. They handle the G98 notification or G99 application, and will tell you if the network is likely to limit export. Get two or three quotes for the same specification.
So: most homes need 8–14 panels, and you find your own number by dividing annual usage by yield, adding room for an EV, heat pump or battery if they are coming, and checking the roof can take it. Run our roof check to see the panel count and kWp your roof supports, put that size into the savings calculator for the annual benefit range it produces, then compare quotes from MCS-certified installers covering your postcode. You contact them directly, on your terms.
A typical three-bedroom home on gas heating, using around 2,700–3,500 kWh of electricity a year, usually lands on 9–12 panels, a system of roughly 4–5 kWp. Add an electric car or a heat pump and that rises to 13–15 panels, or as many as the roof will take. The exact count depends on panel wattage, roof direction and usable roof area, so treat it as a bracket for an installer to confirm.
With current 400–450 W panels, a 4 kWp system is 9 or 10 panels and needs roughly 19–23 square metres of clear, usable roof. On a good, unshaded UK roof it produces roughly 3,400–4,000 kWh a year, broadly what an average home uses; a typical real roof makes somewhat less.
Yes. The 3.68 kW figure is a limit on inverter export under the G98 fit-and-inform route, not a cap on panels. Installers commonly fit 4.5–5 kWp of panels behind a 3.68 kW inverter with little lost over a year. Beyond that, a G99 application to your network operator is needed before installation. In most cases it is approved within a few weeks, sometimes with an export limit attached.
Budget roughly 2–2.2 square metres of clear roof per panel once edge margins are included: about 15–18 m² for a 3 kWp system, 19–23 m² for 4 kWp, 23–29 m² for 5 kWp and 27–33 m² for 6 kWp. Only pitches facing roughly south, east or west count, and chimneys, rooflights and dormers reduce the usable area.
If you expect to buy one within a few years, yes, within reason. An EV adds roughly 2,000–4,000 kWh of annual electricity use, and adding panels later means paying for scaffolding again and possibly a new inverter. The marginal cost of extra panels during the original install is comparatively low. If the EV is a distant maybe, size for today and ask your installer to leave room to expand.
Enough about the theory.
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