Home energy guide
What size solar panel system do I need?
Most UK homes end up with a system somewhere between 3 and 6 kWp. The right size for you comes down to three things: your roof, how much electricity you use (and when), and what you'll be paid for the spare.
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Work out your own numbers with the solar calculator →
Step 1: how much roof have you got?
Solar is sold in kilowatts peak (kWp): what the panels make in perfect test conditions. A typical 4.5 kWp system is about 12 panels and covers 20–30 m² of roof (Energy Saving Trust). As a rough rule, every kWp needs about 4.5–6.5 m² of clear, unshaded roof (our sum from EST's figures).
Leave space for edges, roof windows, vents and chimneys. Your installer will draw the layout.
South-facing is best. East or west roofs make about 15–20% less (EST), and EST doesn't recommend north-facing ones. Splitting panels across an east and a west slope can work nicely, because it spreads your power through the day.
Step 2: how much will it make where you live?
Sunnier, more southerly spots win. Plymouth gets about a quarter more per panel than Glasgow. We ran the EU's PVGIS tool for a few cities (south-facing roof at 35°, 14% system losses):
See the numbers
| Location | Per kWp | 3 kWp | 4.5 kWp | 6 kWp |
|---|---|---|---|---|
| Plymouth | 1,122 kWh | 3,366 | 5,049 | 6,732 |
| London | 1,019 kWh | 3,057 | 4,586 | 6,114 |
| Birmingham | 968 kWh | 2,904 | 4,356 | 5,808 |
| Manchester | 886 kWh | 2,658 | 3,987 | 5,316 |
| Glasgow | 859 kWh | 2,577 | 3,866 | 5,154 |
Somewhere else? Run PVGIS yourself or pick your region in the solar calculator.
Step 3: match it to how you use power
Check your yearly electricity use on a bill or your supplier's app. Ofgem's typical medium-use home uses 2,700 kWh a year (not re-checked today). A small system can make more than that over a year, but mostly in summer and around midday.
That timing matters, because your solar earns money in two very different ways:
- Every unit you use yourself saves you about 26p (the price cap rate).
- Every unit you export earns about 12p from a Smart Export Guarantee tariff.
So extra panels beyond what you can use mostly earn the cheaper 12p rate. Still worth having, just slower to pay back. Going bigger makes more sense when:
- you have (or plan) an EV that's at home in the day;
- you plan a heat pump, which uses more electricity, mostly in winter;
- you add a battery, which saves midday sunshine for the evening;
- someone's home in the day, or you can run appliances and hot water on timers.
Step 4: check the price per kWp
Scaffolding, the inverter, the electrician and the paperwork cost much the same for any size. So bigger systems are cheaper per kWp. If you have the roof, going a bit bigger is often better value, even if more of the extra gets exported.
Planning to add panels later? Ask if the inverter has spare capacity. A second inverter and another round of scaffolding later costs a lot more.
A worked example
Picture a London home that uses 3,500 kWh a year and is out all day. A 4.5 kWp system would bring in about £780 a year; a 3 kWp system about £564. So the bigger one earns roughly £216 more a year. Whether that's worth it depends on how much more it costs in your quotes.
See the numbers
- 4.5 kWp makes about 4,586 kWh. If 35% is used at home: 1,605 kWh × 26.32p = £422 saved. The other 2,981 kWh exported at 12p = £358. Total about £780 a year.
- 3 kWp makes 3,057 kWh. Assume a higher share used at home, 45%: 1,376 kWh = £362 saved; 1,681 kWh exported = £202. Total about £564 a year.
- Import price: 26.32p, the Ofgem cap average for October to December 2026. There's no VAT on home electricity until 30 June 2027 (HMRC VAT Notice 701/19, checked 2 Nov 2026); with 5% VAT it would be 27.64p.
- Export: Outgoing Octopus pays 12p, and the Energy Saving Trust says about 12p is typical.
- These self-use shares are illustrations. Yours depends on your habits.
Export limits
Your network operator may limit how much you can export at once, especially for larger systems or three-phase supplies. Your installer handles the application, but ask early if you're planning something bigger than a typical home system.
Frequently asked questions
How many solar panels do I need for a 3-bed house?
There is no fixed answer, but the Energy Saving Trust says a typical home system is about 4.5 kWp, roughly 12 panels. Size yours from your roof space and electricity use.
How much roof space does a 4 kWp system need?
EST says a 4.5 kWp system covers 20–30 m². A 4 kWp system needs a little less, roughly 18–26 m², depending on panel size.
Is it worth oversizing solar panels?
Often, if you have the roof. The cost per kWp falls with size, and extra output is still worth the export rate (about 12p). It is more worthwhile with an EV, heat pump or battery.
How much electricity will a 4 kWp system generate?
About 4,076 kWh a year in London and 3,436 kWh in Glasgow for a south-facing roof, based on PVGIS figures of 1,019 and 859 kWh per kWp.
Related guides
- Solar panels in the UK: cost, savings and payback
- How much do solar panels cost in the UK in 2026?
- Do you need planning permission for solar panels?
- The Smart Export Guarantee: getting paid for your spare solar
- East, west, flat or shaded: is your roof good enough for solar?
- All home-energy calculators
Sources (checked 27 October 2026 unless stated)
- Energy Saving Trust: Solar panels: 4.5 kWp, ~12 panels, 20–30 m², east/west 15–20% less, about 12p export
- EU JRC PVGIS 5.3 (queried 27 Oct 2026 for our solar guide): kWh per kWp for UK cities
- Ofgem: price cap 1 Oct–31 Dec 2026 (published 26 Aug 2026): 26.32p/kWh
- HMRC VAT Notice 701/19 (updated 28 Oct 2026, checked 2 Nov 2026): 0% VAT on GB domestic electricity 1 Oct 2026 to 30 Jun 2027
- Octopus Energy: Outgoing Octopus: 12p/kWh export
- Ofgem typical domestic consumption values (2,700 kWh): not re-checked today