How this calculator works
A solar panel system is rated in kilowatts peak (kWp): the number of panels times their watts. In a year in the UK each kW makes about 850 to 1,050 kWh, depending on how far south you are, and the output is scaled by the direction of the roof (south is 100%, east or west about 85%). Some of the power is used in the home, which saves the price you pay, and the rest is exported and paid at the export rate. The payback is the cost of the system divided by the benefit each year.
Worked example: ten 400 W panels are 4.0 kW. On a south roof in the Midlands they make 3,800 kWh a year. If 40% is used at home, that is 1,520 kWh saved at 26p (£395), and 2,280 kWh exported at 12p (£274), a benefit of £669 a year. At £1,600 per kW the system costs £6,400, which pays back in 9.6 years. On an east or west roof the output falls to 3,230 kWh, and on a north roof to 2,280. In the south of England it is 4,000 kWh. Using 70% of the power at home raises the benefit to £828 (payback 7.7 years). Six panels make 2,280 kWh and sixteen panels 6,080 kWh.
Solar output by system size (south roof, Midlands, 40% used at home, 26p, export 12p)
Panels of 400 W.
| Panels | System | kWh a year | Roof area | Benefit a year | Payback |
|---|---|---|---|---|---|
| 4 | 1.6 kW | 1,520 | 8 m² | £268 | 9.6 years |
| 6 | 2.4 kW | 2,280 | 11 m² | £401 | 9.6 years |
| 8 | 3.2 kW | 3,040 | 15 m² | £535 | 9.6 years |
| 10 | 4.0 kW | 3,800 | 19 m² | £669 | 9.6 years |
| 12 | 4.8 kW | 4,560 | 23 m² | £803 | 9.6 years |
| 16 | 6.4 kW | 6,080 | 30 m² | £1,070 | 9.6 years |
Tips before you buy
- A south-facing roof at a 30 to 40 degree pitch is best, but east and west roofs make about 85% as much, and spread the output through the day.
- The more of the electricity you use yourself, the more you save: you avoid the full price (about 26p) and not the export rate (often 5 to 15p). Run the dishwasher and the washing machine in the day.
- A battery stores the daytime surplus for the evening, and raises the share you use. It adds to the cost, so work out whether it pays for itself.
- Shade from trees, chimneys and neighbouring buildings can cut the output a lot, even over part of the array.
- Get at least three quotes from MCS-certified installers, and check the panels, the inverter, the warranty and the monitoring.
Tools & materials for this project
Affiliate links As an Amazon Associate we earn from qualifying purchases. These links do not change what you pay.
Frequently asked questions
How much electricity does a solar panel system make in the UK?
A 4 kW system (ten 400 W panels) on a south roof makes about 3,800 kWh a year in the Midlands, 4,000 kWh in the south of England and 3,600 in the north. East and west roofs make about 3,230 kWh.
How many solar panels do I need?
A typical home uses about 2,700 kWh of electricity a year, which six 400 W panels (2.4 kW) can nearly cover on a south roof: 2,280 kWh. Most homes fit 8 to 12 panels, which is 3.2 to 4.8 kW.
How long do solar panels take to pay back?
About 10 years for a 4 kW system at £1,600 per kW, if 40% of the power is used at home. Using more yourself, with a battery, an electric car or a heat pump, shortens it.
How much roof area do solar panels need?
About 1.9 m² per panel, so ten panels need about 19 m² of roof, in a layout that fits the shape of the roof.
Do solar panels work on a north-facing roof?
Yes, but they make much less: about 2,280 kWh a year for the same ten panels, which is 60% of a south roof. East and west roofs are a better choice.
Add this calculator to your website
Running a blog, a contractor site or a class page? You can put this calculator on it for free. Copy the code below, and please leave the link underneath the calculator in place. The small script just resizes the frame to fit the results; if your site strips scripts, the calculator still works inside a scrolling frame.
Related calculators & guides
Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.