A 4 kW solar system: cost and output
Four kilowatts is the size the UK domestic market settled on, and the one most cost comparisons quote. At the 2025/26 median it is about £6,400 installed — a figure that comes from what tens of thousands of households actually paid.
| Roughly how many panels | 10 at 430 W |
|---|---|
| Published size band | 0-4 kW |
| Installed cost, median | £6,380 (£1,595/kW) |
| Installed cost, mean | £7,234 (£1,808/kW) |
| Generation, south-facing at 35° | 3,424 kWh a year for a median location; 2,768 to 4,168 kWh across our 25 reference locations |
Cost: Solar photovoltaic (PV) cost data, Department for Energy Security and Net Zero, 2025/26, Total Installed Capacity basis, OGL v3.0. Generation: our own PVGIS modelling at 20% system losses — see generation by region. Panel count is illustrative and depends on the modules quoted.
| Self-consumption | Used at home | Exported | At 12.0p export | At 4.1p export |
|---|---|---|---|---|
| 14% | 479 kWh | 2,945 kWh | £479 | £246 |
| 19% | 651 kWh | 2,773 kWh | £503 | £284 |
| 23% | 788 kWh | 2,636 kWh | £522 | £314 |
Self-consumption rates are MCS guidance note MGD 003's figures for a household using 2,500–2,999 kWh a year — 14% out all day, 19% in half the day, 23% home all day. They are averages for similar households, not a prediction for yours, and self-consumption falls as the array gets larger. Import is the price cap unit rate for 1 July to 30 September 2026. The two export columns are the best and the median open untied rate in our SEG table — the gap between them is what choosing an export tariff is worth, and it is larger than most people expect.
Who this size suits
- typical three-bedroom houses with a usable south, east or west roof plane
- households using roughly 2,500 to 3,500 kWh a year
- anyone wanting to compare a quote against the most widely published benchmark
Roughly 10 panels at 430 W, needing about 20 m² of roof. Higher-rated modules need fewer panels for the same capacity, which matters on a tight roof.
Why 4 kW became the default
Partly the roof — four kilowatts fits a typical semi-detached roof plane without difficulty. Partly history: the Feed-in Tariff’s highest domestic rate band ran to 4 kW, and the market shaped itself around it for a decade. The subsidy is long closed to new applicants; the habit remains.
That makes 4 kWp the most quoted size, the most compared, and the easiest to sanity-check. Which is a reason to use it as a reference — and not a reason to assume it is right for your house.
What actually decides whether 4 kW is right
Your consumption, not your roof. At 4 kWp on a median location our modelling puts generation at roughly 3,400 kWh a year facing south. A household using 2,700 kWh cannot use most of that at the moment it is produced — see self-consumption.
Your roof orientation. The figures above are south-facing. East–west splits produce less in total but spread it across the day, which can raise the share you use. That trade-off is worked through in east–west versus south facing.
Whether you will add a battery or an EV. Both change the arithmetic substantially, and both argue for more capacity rather than less.
Checking a 4 kW quote
Divide the price by the capacity and compare with the benchmark. Above the mean is not automatically wrong — access, roof type and electrical work all move it legitimately — but it should come with a reason.
Check the quote, not the brochure
Divide the quoted price by the array’s kWp and compare it with the published benchmark. Then check what the quote includes, and what it does not — the costs outside the headline are where budgets break.
Sources
- Solar photovoltaic (PV) cost data Release 2025-26, published 28 May 2026, Total Installed Capacity basis. Derived by the department from the MCS Installation Database.
- MGD 003: Solar PV Self-Consumption, issue 2.0 (1 April 2022) Read locally. MCS restricts reproduction, so we cite clauses and quote sparingly.
- Energy price cap unit rates and standing charges
Contains public sector information licensed under the Open Government Licence v3.0.
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