A 3 kW solar system: cost and output
Three kilowatts fits most modest roofs and suits a household that is not at home much during the day — because at this size, a larger share of what you generate gets used rather than exported.
| Roughly how many panels | 7 at 430 W |
|---|---|
| Published size band | 0-4 kW |
| Installed cost, median | £4,785 (£1,595/kW) |
| Installed cost, mean | £5,425 (£1,808/kW) |
| Generation, south-facing at 35° | 2,568 kWh a year for a median location; 2,076 to 3,126 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% | 360 kWh | 2,208 kWh | £359 | £184 |
| 19% | 488 kWh | 2,080 kWh | £377 | £213 |
| 23% | 591 kWh | 1,977 kWh | £391 | £235 |
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
- terraced and smaller semi-detached houses with one usable roof plane
- households using around 2,000 to 2,700 kWh a year
- anyone whose roof has obstructions that break up the available area
Roughly 7 panels at 430 W, needing about 14 m² of roof. Higher-rated modules need fewer panels for the same capacity, which matters on a tight roof.
Why smaller arrays self-consume a larger share
This is the fact that decides whether 3 kWp or 5 kWp is the better buy, and it is counter-intuitive.
MCS’s guidance note MGD 003 shows self-consumption falling as generation rises for the same household. For a home-all-day household it gives around 29% at roughly 2,550 kWh of annual generation, 23% at 3,750 kWh and 17% at 5,850 kWh.
The reason is obvious once stated: your daytime demand is what it is. A bigger array does not create more daytime demand — it produces more surplus, and surplus earns the export rate rather than saving you the import rate.
So the question is not “how big is my roof”. It is “how much of the extra will I actually use?” — and the answer depends on your household, not your roof.
Where 3 kW sits
Comfortably inside the 0–4 kW published band, and comfortably inside G98, so your installer notifies the network operator after installation rather than applying first.
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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