How much can a 4kW solar system save in the UK?
This worked example applies the solar savings formula to one common UK planning size. A 4kW system does not have one fixed saving: annual generation, household self-use, import and export rates, installed cost and any battery all move the answer.
In the middle example below, a 4kW system produces 3,600kWh a year and gives an estimated annual benefit of £680. That is a planning example, not a guarantee. Your roof, tariff, export rate and quote can all move the answer.
Quick answer
A sensible 4kW UK planning range is about 3,200 to 4,000kWh of annual generation. With the current default assumptions on this site, the middle 3,600kWh case gives about £680a year in combined bill savings and export income.
4kW savings scenarios
| Scenario | Key assumptions | Annual benefit | Payback | 20-year net |
|---|---|---|---|---|
| Careful | 3,200kWh generation, 30% self-use, 8p export, no battery. | £430 | 16.3 years | £1,595 |
| Middle | 3,600kWh generation, 35% self-use, 15p export, no battery. | £680 | 10.3 years | £6,597 |
Middle-case calculation, step by step
| Calculation | Result |
|---|---|
| 3,600kWh generation × 35% self-use | 1,260kWh used at home |
| 1,260kWh × 26.1p import rate | £329 bill saving |
| 2,340kWh × 15p export rate | £351 export income |
| Bill saving + export income | £680 annual benefit |
| £7,000 installed cost ÷ annual benefit | 10.3 years simple payback |
This is deliberately narrower than the general UK solar savings guide: it holds system size at 4kW so you can see the effect of changing the assumptions.
Why your result may differ
- A shaded or awkward roof can produce much less than a clean south-facing roof.
- High daytime usage improves bill savings because more solar is self-consumed.
- A strong export tariff can make exported kWh more valuable.
- A battery can reduce later grid imports, but it also has losses, can displace export income and adds its own installed cost.
How to run your own 4kW case
- Set system size to 4kW.
- Enter a realistic annual generation estimate, such as 3,200 to 4,000kWh.
- Use your own import unit rate and export tariff.
- Complete the solar-only case first. Then use the current battery comparison with the quoted usable battery capacity and incremental installed battery cost.
Start here: run your own 4kW estimate · check the calculation method · generation by system size · battery vs no battery