How your estimate is worked out

There is no black box here, and it is worth knowing the shape of the sum so you can tell which of your answers is doing the work.

Everything starts with the installer’s own figures

Before any of this runs, the installer fills in a sheet of assumptions: how much electricity a system generates in their part of the country, what they charge, what they think a household typically uses of its own solar, and what the local export payment is. Your answers are then run through their numbers.

This is the reason two installers can give the same house different estimates without either of them being wrong. It is also why an estimate is only ever as good as the sheet behind it.

The chain, step by step

1. How big a system fits

Your roof size gives a usable area in square metres, either from the band you picked or from the figure you typed. That area is turned into a system size, because panels take up a known amount of room per kWp. A flat roof fits less than its area suggests, because panels sit on frames with gaps between the rows.

Separately, the calculator works out how big a system your electricity use would justify. It then takes the smaller of the two, and rounds to a whole number of panels.

If the roof is the smaller of the two, that is when step 4 tells you your roof is the limit rather than your usage.

2. How much it would generate

Generation is the system size multiplied by three things:

Yearly generation per kWp, the installer’s own figure for their region. Across the UK this sits at roughly 850 to 950 kWh per kWp a year, higher in the south.
An orientation factor. South is the full figure. The further round from south your roof faces, the lower it gets, with east and west losing roughly a fifth. If you answered “Not sure”, no penalty is applied and south is assumed, which the estimate states rather than hiding.
A shading factor, from your answer about trees, chimneys and neighbours. More shade, less generation.

That gives the kWh a year quoted in the sentence under your four numbers.

3. How much of it you actually use

This is the part most people have never thought about, and it matters more than almost anything else.

Solar generates in the middle of the day. Most households use most of their electricity in the morning and the evening. So a good share of what you generate is not needed at the moment it appears, and goes back to the grid instead.

The calculator splits your generation in two:

Used at home. Worth your full unit rate, because it is electricity you did not have to buy.
Exported. Worth the export payment, which is markedly less.

The share used at home is one of the installer’s assumptions, and it is the single figure a battery changes: storing the day’s surplus for the evening moves electricity from the cheap column to the valuable one. There is a sensible cap on it, too. A household can never use more than it actually uses, so a very large system on a low-consumption house does not get credited with impossible savings.

Your yearly saving is those two amounts added together.

4. What it would cost

The installation cost is built up from the installer’s own costs: a base figure covering everything that does not scale with panel count (survey, network paperwork, certification), plus an amount per kWp of system, plus scaffolding, plus the battery and any extra for mounting it outside.

The per-kWp rate is banded, so it falls as the system grows.

5. Payback

Cost divided by yearly saving. Simple, not discounted.

How your answer about energy use is turned into kWh

Whichever way you answered at step 3, it ends up as a single figure for the year:

You said	What happens
A yearly kWh figure	Used exactly as you gave it
A monthly bill	Multiplied by twelve, then divided by your unit rate
A band	A typical figure for that band, adjusted for the size of your property
I don’t know	A typical figure for a home your size, and your enquiry is flagged so the installer knows to confirm it

If you gave both a day rate and a night rate, they are blended, weighted by how much electricity a household typically uses in the daytime rather than split down the middle. Most usage is daytime usage, so the day rate carries more of it.

Next: the instalment plan, or what the estimate is not.