A solar panel calculator turns three numbers into a system size: your yearly electricity use, your roof’s output and your roof space. You can do the sums by hand in about ten minutes, since the method needs only a bill and a tape measure. This guide shows the method step by step, with a worked UK example.
Knowing your size before you ask for quotes helps you spot a system that is too big or too small, because installers sometimes quote for more panels than you need. Prices below are as at October 2026 and include 0% VAT.
The solar panel calculator formula
Here is the whole method in one line.
System size (kWp) = yearly electricity use (kWh) ÷ yearly output per kWp (kWh)
For a typical UK roof we assume about 900 kWh per kWp a year. As a result, a home using 3,600 kWh needs about 4 kWp. Each step below explains where the two numbers come from and how to adjust them.
Step 1: Find your yearly electricity use
Your bill or supplier app shows your yearly use in kWh. If it does not, add up twelve months of readings, because a full year captures every season. A smart meter app usually shows this in a few taps.
If you have no bill to hand, start from Ofgem’s typical figures. A low-use home uses about 1,800 kWh a year, a medium home about 2,700 kWh and a high-use home about 4,100 kWh. Your own bill is still better than an average.
Now add anything you plan to run later, for example a heat pump or a car. A heat pump in a typical home uses roughly 3,500 kWh a year, which is our estimate rather than a measured figure. For an electric car, divide your yearly miles by the car’s miles per kWh, which the maker states. Our guide to charging an EV from solar covers that in more detail.
Step 2: Pick the right yield for your roof
Yield is the electricity one kWp of panels makes in a year. It depends on where you live and which way your roof faces, so two homes can differ a lot. For a typical UK roof we use 900 kWh per kWp, which is deliberately cautious.
| Your roof | Yield to use (kWh per kWp per year) |
|---|---|
| South-facing, southern England, no shade | About 1,000 |
| Typical roof (our standard figure) | 900 |
| East or west facing, or partly shaded, in the north | About 750–800 |
The EU’s PVGIS solar model gives higher figures for an ideal roof: due south, tilted at 35°, with no shade. Real roofs rarely match it. Therefore, treat these as the ceiling. Each figure is for 1 kWp.
| Place | PVGIS yield, ideal roof (kWh per kWp) | A 4 kWp system makes (kWh a year) |
|---|---|---|
| London | 1,025 | About 4,100 |
| Cardiff | 1,030 | About 4,120 |
| Birmingham | 970 | About 3,870 |
| Belfast | 920 | About 3,670 |
| Edinburgh | 915 | About 3,660 |
| Leeds | 945 | About 3,790 |
| Manchester | 885 | About 3,540 |
| Glasgow | 850 | About 3,400 |
For a closer look, see solar panels in London, Scotland solar costs and support and Northern Ireland solar costs and support. Each page uses local yield figures, since yield varies by place.
Orientation matters too. London figures, compared with a south-facing roof at 35°:
| Direction | Share of south-facing output |
|---|---|
| South-east | About 95% |
| South-west | About 93% |
| East or west | About 78–82% |
| Flat (no tilt) | About 83% |
| North | About 53% |
Anything from 20° to 45° tilt costs you no more than about 4%, so a steep or shallow roof is rarely a problem. Our guide to how orientation affects output goes through the options.
Step 3: Work out your system size
Now divide, while keeping your yearly use in kWh on top and your yield underneath.
- 2,700 kWh ÷ 900 = 3.0 kWp
- 3,600 kWh ÷ 900 = 4.0 kWp
- 4,500 kWh ÷ 900 = 5.0 kWp
- 3,500 kWh on an east-west roof in the north: 3,500 ÷ 775 = 4.5 kWp
The result is the size that would make about as much electricity as you use over a year. However, you will not use all of it at the time it is made. Without a battery, our standard model assumes you use about 30% yourself and export the rest.
For that reason, many households do not size to 100% of their use. Roof space and budget often decide the final size. A bigger system still helps, because exported power earns money.
Step 4: Convert kWp into panels and roof space
Divide your system size by the panel’s rating. Most panels sold for UK homes today are rated about 430–450 W, although older panels were weaker. A typical panel measures about 1.76 m by 1.13 m, which is close to 2 m² each.
| System size | Panels (at 440 W) | Roof area needed |
|---|---|---|
| 3 kWp | 7 | About 14 m² |
| 4 kWp | 9 or 10 | About 18–20 m² |
| 5 kWp | 11 or 12 | About 22–24 m² |
| 6 kWp | 13 or 14 | About 26–28 m² |
Measure the usable part of your roof, not the whole slope. Chimneys, roof windows and the edges all take space, so the usable area is smaller than the whole roof. Your installer will confirm the exact layout during the survey. For a fuller walk through the count, read how many panels a typical home needs.
Step 5: Estimate your yearly saving
Next, turn kWh into pounds, because the bill is what matters. Use three inputs: the price you pay, the rate you earn for exports and the share you use at home.
- The Ofgem price cap for October to December 2026 averages 26.32p per kWh for electricity.
- Our standard export rate is 12p per kWh, which suppliers such as Octopus and OVO pay to their own electricity customers. Open-market rates are about 3–6p.
- Standing charges do not change, because solar does not reduce them.
For example, here is the sum for one kWp, using 900 kWh a year and 30% used at home:
- Electricity you use: 270 kWh × 26.32p = about £71
- Electricity you export: 630 kWh × 12p = about £76
- Total: about £147 per kWp a year
A 4 kWp system therefore saves and earns roughly £590 a year. Larger systems export more, so their saving per kWp is slightly lower. With open-market export at 4.1p, the same 4 kWp system makes about £390.
Since the same model drives our cost pages, see typical UK solar costs and payback. Rates are reviewed quarterly, so check the latest cap before you rely on these figures.
A worked example, start to finish
For example, take a three-bedroom semi that uses 3,500 kWh a year, with a typical roof.
- Yearly use: 3,500 kWh, read from the bill.
- Yield: 900 kWh per kWp, our standard figure.
- Size: 3,500 ÷ 900 = 3.9 kWp.
- Panels: 9 panels at 440 W gives 3.96 kWp.
- Roof space: 9 × 2 m² = about 18 m².
- Yearly output: 3.96 × 900 = about 3,560 kWh.
- Yearly saving: 3.96 × £147 = about £580.
The installed cost of a 4 kWp system is about £6,500–£8,500. On our standard model, that pays back in about 13 years, or roughly 11–15 years depending on price, roof and export tariff. See a full breakdown of installation prices for the cost of the size you land on.
Sources: Ofgem typical domestic consumption values; Ofgem price cap, October to December 2026; EU PVGIS; DESNZ solar PV cost data.
Check the result against real data
A calculator gives a good first estimate. However, a survey is better, because an installer can see shading and roof condition. Ask for the MCS estimate of yearly output, and compare it with your own figure.
If the two differ by more than about 10%, ask why, because one of the inputs may be wrong. Our guide to how much a system generates month by month shows what to expect through the year. Output in winter is far lower than in summer, so a yearly total hides a big seasonal swing.
A calculator is a good start. When you are ready, ask for free quotes from local installers and give them your size and roof details.
Frequently Asked Questions
Q: What is the quickest way to size a solar system? A: Divide your yearly electricity use in kWh by 900. The answer is the system size in kWp. For example, 3,600 kWh gives 4 kWp, or 9 to 10 panels.
Q: Why use 900 kWh per kWp rather than a higher number? A: PVGIS figures assume an ideal south-facing, unshaded roof. Real roofs face other ways and often have some shade. As a result, 900 is a cautious figure for a typical UK roof.
Q: Does a bigger system always save more? A: It earns more in total, but each extra panel adds less. You use only part of what you make at the time, so the rest goes to the grid at the export rate. A battery raises your own use, though it lengthens payback on a flat-rate tariff.
Q: Does an east-west roof need a bigger system? A: Usually, yes. East- and west-facing panels make roughly 80% of a south-facing array. They do spread output across the morning and evening, which can suit homes with people in all day.
Q: Should I size for a future EV or heat pump? A: If you will buy one within a year or two, it is worth planning for. Add its yearly electricity to your total in Step 1. Check that your roof has room, because adding panels later costs more per panel.