Every quote contains a generation estimate in kWh per year. It is the number every savings figure is built on, so it is worth knowing how to read it.
A realistic figure
As a rough guide, 1kW of panels produces around 850 to 1,000 kWh a year in the UK, for a reasonably oriented, unshaded roof.
So:
| System size | Rough annual generation |
|---|---|
| 3kW | 2,550 to 3,000 kWh |
| 4kW | 3,400 to 4,000 kWh |
| 5kW | 4,250 to 5,000 kWh |
Anything materially above that range for a standard installation deserves a question. Anything well below it suggests shading or a difficult roof, which should be explained in the quote.
What the estimate should account for
A proper estimate is produced using the standard methodology and takes in:
- Location. Southern England receives more than northern Scotland, though the gap is smaller than most people assume.
- Orientation. South is the reference. East and west typically lose around a fifth.
- Pitch. Most UK pitches are fine. Very shallow or very steep costs a little.
- Shading. Should be measured, not eyeballed.
- Panel specification.
- System losses. Cabling, inverter efficiency, temperature, dirt.
If an installer cannot say what shading factor they applied, the estimate is not reliable.
How to spot an optimistic estimate
- No shading factor on a roof that visibly has shading.
- A figure well above 1,000 kWh per kW without a specific explanation.
- The same figure for two different roofs on the same house.
- A round number. Real calculations rarely produce exactly 4,000.
- No mention of the methodology used.
Generation is not the same as saving
This is the part that matters.
Your saving does not come from generating. It comes from using what you generate, or being paid for exporting it. A unit used saves about 26.32p. A unit exported earns roughly half.
So when a quote turns generation into a pounds figure, the important question is: what self-consumption percentage did you assume? Ask it. Then ask what the figure looks like ten percentage points lower.
See payback and how much you will export.
What it will look like month to month
British solar is seasonal, and more so than people expect. A typical system produces a large majority of its annual output between April and September. A December day may produce a tenth of what a June day produces.
This matters for expectations. The first winter after installation is when people ring their installer convinced something is broken. Usually nothing is.
It also matters for heat pumps, where demand peaks exactly when generation does not. See solar and a heat pump together.
Checking it afterwards
Keep the estimate. Compare it against your monitoring after a full year, not after a month. A single month tells you about the weather. A year tells you about the system.
If the shortfall is more than about 10% over a full year, raise it with the installer.