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Learn · Solar PV
Five things decide what a solar array makes in a year. Here is each one, what it does, and what it does for your roof — change anything as you go.
A panel is sold by its kilowatts peak — 4.32 kWp for twelve 360 W modules. That is not a promise about a year. It is what the array produces in a laboratory, under a standard test: bright, perpendicular light and a cool panel.
Britain supplies none of those things reliably, so the whole job of this calculator is turning the first number into the second.
The same array does not produce the same energy everywhere in Britain. The method divides the country into 21 regions — 25 once four of them split by nation — and your postcode picks one.
Your roof's exact pitch and orientation, moved around the country:
Real, but smaller than people expect — the difference between the sunniest and least sunny parts of Britain is about a third. Which way your roof faces matters more.
Two numbers describe a roof plane: its orientation, measured as degrees away from due south, and its pitch, measured from horizontal. The method rounds them — orientation to the nearest 5°, pitch to the nearest 1° — before looking anything up.
Note what orientation is not: it does not distinguish east from west. Both are 90° from south, and over a whole year they collect about the same.
Notice that the best pitch changes as the roof turns away from south — flatter roofs care less about direction, because they see more of the whole sky.
Everything so far came from a table. Shading is the one input that comes from looking — and it is the least certain part of the whole estimate.
The method divides the sky into 84 segments. You sketch the horizon as seen from the array — the roofline opposite, the tree, the chimney — and every segment below that line costs one hundredth of the output.
Click the chart to raise or lower the horizon in each direction. Doing this properly means standing at the array with a compass and an inclinometer — which is why a shaded estimate made from a desk should be treated as provisional.
The region, the pitch and the orientation collapse into a single figure — kWh per kWp, how much energy a kilowatt of panel yields here, on this plane, in a year. Multiply by the array size and by the shade factor, and that is the estimate.
We show that as ~3,950 kWh, not 3948.48. Both are the same calculation. The second one just looks more certain than it has any right to.
More decimal places make a number more precise. They do not make the estimate behind it more accurate. Honestly stated, this array produces somewhere between 3,600 and 4,300 kWh in a typical year — and a cloudy year or a sunny one will move it further than that.
That figure comes from the recognised lookup tables, so it is what a certified installer will quote you. Those tables were built from a satellite dataset that has since been retired. Run exactly the same calculation on the data that replaced it and this roof gives 3,750 kWh — 5% lower.
Same method, same roof, same arithmetic. The only thing that changed is which years of satellite measurements the table was built from. Neither number is wrong, and the distance between them is a fairer picture of what anyone can really tell you than either one on its own.
Everything above answers one question: how much electricity comes off the roof. It says nothing about how much of it you actually use — and that depends on when it arrives, which is a different problem with a different method.
Two homes using exactly the same number of units a year can get very different value from the same array — because solar arrives at noon and most households do not. That is a separate method, and it carries on from here with the figures you have just set.