By the Home Savings Calculator team · 29 August 2026 · 7 min read
A twenty-five year savings total can be made to look impressive on almost any roof — stretch the horizon far enough and every system pays. Payback period is the honest measure: how many years until the system has returned what it cost.
Under ten years is strong. Ten to fifteen is reasonable if you are staying put. Beyond fifteen, on a twenty-five year panel life, you are making a thin bet on electricity prices and on not moving house.
Put the quoted system size and installed price into the solar panel calculator. If the payback year is far from what the quote implies, the difference is usually optimistic assumptions about how much you export and what it earns.
Orientation is the single biggest physical factor. Against a due-south roof, south-east or south-west loses only a few percent, east-west around fifteen, and north closer to forty. That is the difference between a good investment and a poor one, and no amount of panel quality recovers it.
Shading matters more than people expect, because panels are wired in strings — one shaded panel can drag down its neighbours. A chimney or a neighbour's tree that clips the array for two hours a day is worth asking about before signing.
A unit you use in the house saves you the full import rate. A unit you export earns the export rate, typically a third of that or less. So the same generation is worth wildly different amounts depending on when you use electricity.
This is why solar suits people who are home during the day, run heating or hot water on electricity, or charge an electric car on the drive. If the house is empty from eight till six, most of your generation leaves at the low rate and payback stretches accordingly.
Check the price includes scaffolding, the inverter, and the electrical work — some quotes carve these out and reappear with them later. Check the assumed annual generation against the system size; a figure far above roughly 850–950 kWh per kWp in northern Europe deserves a question.
Check the export rate the projection assumes, and whether you can actually get it from your supplier. An optimistic export rate quietly improves every downstream number in the quote.
A battery raises self-consumption by storing midday generation for the evening, which is genuinely valuable — but it adds several thousand to the price and has a shorter life than the panels. On running costs alone it often lengthens payback rather than shortening it.
It stacks up better if you also have a time-of-use tariff, because the battery can charge cheaply overnight as well as from the roof. Model it by raising the self-consumption assumption and adding the battery to the install cost.
