Solar panel yield and payback calculator

A solar quote arrives as one number — the size of the array in kilowatt-peak — and every question you actually have depends on two things the quote does not contain: how much sun falls on that particular roof, and how much of the output you happen to be at home to use. The first is geography. The European Commission's PVGIS puts 1 kWp tilted 35° due south at 1082.5 kWh a year in Kraków, 1057.54 in Berlin, 1018.88 in London and 1618.87 in Madrid: the same panels, half as much again. Turn that Kraków array to face east instead and it drops to 849.36; lay it flat, 899.45; hang it vertically on the wall, 775.4. This page takes the yield for your own roof, the share you use rather than export, and the two prices from your own paperwork, and gives you the year and the payback. It ships no tariff and no yield of its own, because neither one is a constant.

How it is calculated

kWh = kWp × yield · value = used × price + exported × export price · payback = cost ÷ value

Kilowatt-peak is what the array makes under standard test conditions — PVGIS defines those as 1000 W/m² in the plane of the array at 25 °C — so it is a rating, not an output. Multiply it by the specific yield for your location and orientation, in kilowatt hours per kWp per year, and you have the year. Split that year by the share you use on the spot: those kilowatt hours are worth the price you would otherwise have paid, while the rest are worth whatever your export arrangement pays, which is usually far less. Divide the installed cost by the value of one year and you have simple payback.

Source: PVGIS user manual (European Commission, JRC) — the peak power is what the array produces under standard test conditions, 1000 W/m² in the plane of the array at 25 °C, entered in kilowatt-peak

Questions people ask

Where do I get the yield for my own roof?

PVGIS, run by the European Commission's Joint Research Centre, is free and covers Europe, Africa, most of Asia and the Americas. Drop a pin on your roof, set the slope and the azimuth, enter 1 kWp and read E_y — that is the number this page wants. The figures quoted here all use PVGIS 5.3 with the PVGIS-SARAH3 database, hourly data from 2005 to 2023, free-standing crystalline silicon, and the 14 % system losses PVGIS applies by default. Change any of those and the answer moves.

What does orientation actually cost me?

At 50° north it is worth more than people expect. Taking Kraków at a 35° tilt, PVGIS gives 1082.5 kWh per kWp facing south, 849.36 facing east and 841.13 facing west — roughly a fifth of the year gone. A flat roof gives 899.45 and a vertical south wall 775.4. What that does not tell you is when the energy arrives: east and west arrays spread output across the morning and evening, which can raise the share you use at home even as it lowers the total.

Why does self-consumption matter so much?

Because the two prices are rarely close. A kilowatt hour used the moment it is made saves you the full retail price including whatever levies sit on it; the same kilowatt hour exported earns the export rate, which in most markets is a fraction of that. Move the self-consumption slider on this page and watch the payback move with it — for many households it matters more than another panel would.

Is simple payback the right measure?

It is the honest floor, not the whole answer. It ignores that modules lose output slowly over their life, that an inverter is likely to need replacing before the panels do, that electricity prices move, and that money spent today could have earned something elsewhere. Every one of those needs an assumption about the future, and an assumption dressed up as a decimal is worse than an obvious approximation. If you want a pessimistic reading, run it again with a lower yield and a lower import price.

Does the page account for a battery?

Not directly, and a battery does not change the yield — it changes the share. A battery moves kilowatt hours from the export column to the self-consumed column, so model it by raising the self-consumption figure and adding the battery to the installed cost. That will tell you whether the shift is worth paying for at the two prices you actually face.

Sources

The documents this page reads its numbers out of, linked so you can check them yourself.

1. PVGIS user manual (European Commission, JRC) — the peak power is what the array produces under standard test conditions, 1000 W/m² in the plane of the array at 25 °C, entered in kilowatt-peak
2. PVGIS 5.3 (European Commission, JRC), PVcalc with PVGIS-SARAH3 2005–2023 and 14 % system loss — Kraków 50.06 N 19.94 E, 1 kWp fixed at 35° facing south, E_y = 1082.5 kWh a year

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