Estimate
How much will my solar panels produce?
A worked estimate from published yield data for 38 locations across six continents, with every step shown. No email, no sign-up, nothing hidden. Enter your own electricity prices in your own currency; we never convert, and we never guess what you pay.
Estimated annual generation
3,322 kWh
Glasgow, United Kingdom. Weather alone moves this by about 3.2% either way from one year to the next, before anything else is uncertain.
What that is worth, at 35% self-consumption
Enter what you pay for a unit of electricity, on the left, and this becomes a number. We will not guess it: electricity prices differ by more between two households in one country than between two countries.
The working
Every step, so you can check it on paper or redo it with different assumptions.
- 1830.6 kWh/kWpStart from published yield for your locationGlasgow, facing the south at 35°, from PVGIS PVGIS-SARAH2
- 2× 1.000Adjust for which way the roof facesFacing south (best) is 0%, measured at Manchester
- 3× 1.000Adjust for the pitch35° is 0% against 35°
- 43,322 kWhMultiply by the array size831 kWh/kWp × 4 kWp
- 51,163 kWh / 2,160 kWhSplit it into what you use and what you export3,322 kWh × 35% used, 65% exported
What this estimate cannot do
- It assumes nothing shades the roof. A chimney, a flue or a tree is not in these numbers, and object shading is the largest single reason a real array underperforms its model.
- It uses the nearest city we hold, not your address, and orientation adjustments measured at Manchester for latitudes above 48°. For an exact figure, put your own coordinates into PVGIS.
- It combines the orientation and pitch adjustments by multiplying them, which is an approximation. They are not strictly independent.
- It says nothing about whether a battery is worth buying, because that depends on when you use electricity rather than how much.
- It is not calibrated. There is no lower bound here, and you should not plan against the headline number.
Yield from PVGIS 5.2 (European Commission Joint Research Centre), PVGIS-SARAH2, 2005–2020 average, 14% system loss, roof-mounted crystalline silicon. Retrieved 2026-09-12.
How to work it out yourself
Solar output estimates look like a black box and are not one. The whole calculation is four numbers multiplied together, and you can do it on the back of an envelope once you know where each one comes from.
Yield for your location
How many kilowatt-hours a year one kWp produces where you live, facing the equator at a sensible pitch. PVGIS publishes this free for most of the world. It runs from about 830 in Glasgow to about 1,570 in Denver and 1,500 in Sydney.
Your roof, against that baseline
Turning away from the equator costs surprisingly little at first: 30° off is about 3% almost everywhere. Pitch matters even less. Due east or west is the first serious penalty, at around a fifth, and facing away from the equator costs between 43% and 52% depending on your latitude.
Your array size
In kWp, from the installation certificate. Multiply the adjusted yield per kWp by it and you have annual generation. This is the number an installer quotes, and dividing theirs by your kWp is how you check it.
What a unit is worth
A unit you use saves you the retail price. A unit you export earns your export rate, typically a fraction of it. So the split between the two decides the money, and it is the figure almost no quote states.
Wide photograph of a domestic pitched roof carrying a solar array, shot in flat daylight so individual modules and the mounting rails are legible.
When an estimate is not enough
This page will not tell you whether a battery pays for itself, and it will not give you a figure to plan against. Both need a model of when you use electricity rather than how much, a calibrated lower bound rather than a headline, and the export rules of the place you live. We have built those rules for Great Britain so far, and the coverage page says plainly where else we have not.