How we size a solar and battery system from your bill, not from your roof

The commonest mistake in a solar quote is sizing the system to the roof: as many panels as will fit, and a battery from the price list. The roof is a constraint, not a target. A system is sized to the house underneath it, and that comes down to five numbers. Here is where each one comes from and what it does.

1. How much electricity the house uses in a year

Your bills give it — the annual kWh, or twelve months of statements added up. If you have a smart meter that will share half-hourly data, better still, because then we can see when the house uses it, not just how much. This number sets the ceiling: a system that makes far more than the house can use is exporting at the export rate, and the export rate is the smallest number on this list.

2. What the roof will make

Two things decide it: where you are, and which way the roof faces. Our calculator uses the MCS published figure for the South East — 857 kWh a year for every kilowatt-peak installed — as a deliberately conservative base, and scales it by orientation: south 1.0, south-east or south-west 0.94, an east–west split 0.83, north 0.62. On the regional pages we show the PVGIS model for a south-facing roof at a 35-degree pitch with 14% system losses, which is higher — about 1,037 kWh per kWp at Dorking, 1,125 at Washington and 1,177 at Lewes — and the survey models your actual roof, with its shade, between the two. The point is that the same panel makes a different amount in Dorking and in Lewes, and a different amount again on an east slope, so a quote that does not say which figure it used has not sized anything.

3. How much of it you will use yourself

Without storage a typical home uses somewhere around half of what its panels make; our model assumes 48%. The rest is exported. With a battery sized to the evening load the model moves to 85%. That shift is where most of the money in a battery lives, which is why the next number matters.

4. The evening load

This is the number a battery is sized to — not the array, not the house, but how much the house draws after the sun has gone. We read it from half-hourly data where the meter will give it and from the bills where it will not. An oversized battery never fills; an undersized one is empty by nine. If the evening load is small, the honest battery is a modest one, and if the arithmetic does not support a battery at all, the quote says so and saves us both an afternoon.

5. The car, and anything else on the horizon

An electric car charged overnight is the largest single load most houses will ever add; our model allows 1,800 kWh a year for one. A heat pump is another. A system sized before either arrives is the wrong system for the house that has them, so we ask at the survey and size for the house you will have.

What the money side looks like

The figures we publish are the client’s own and carry their own qualifiers: most domestic installations typically start from around £4,000 to £6,000 depending on the size of the home and how much energy it uses, and UK homeowners on average save £300–£600 a year, sometimes more with a battery or by working from home during the day. The calculator’s modelled basis sits a little above the top of that starting range, and it says so wherever it prints a figure; your own price comes from the survey, not from a model. The export side uses the Smart Export Guarantee rate current in our tariff reference — 12p per kWh as at June 2026 — against a 26.11p unit rate, which is the whole case for keeping a unit rather than selling it.

Where to go next

Run your own numbers through the savings calculator, and read how accurate is the calculator? before you trust it too far. Is a home battery worth it? is the battery half of the sums; SEG tariffs: what exporting pays is the export rate in detail; solar, a battery and an electric car is what the car changes; scaffolding, access and the survey is where the five numbers are measured; the terms are in the glossary. The survey itself is free — what a free survey involves — and it is where the five numbers stop being estimates.