A 3.5 kW solar system: cost and output
Three and a half kilowatts is not a natural roof size. It exists because of a number in an engineering recommendation: 3.68 kW, the largest inverter that stays inside the simpler network process.
| Roughly how many panels | 9 at 430 W |
|---|---|
| Published size band | 0-4 kW |
| Installed cost, median | £5,583 (£1,595/kW) |
| Installed cost, mean | £6,330 (£1,808/kW) |
| Generation, south-facing at 35° | 2,996 kWh a year for a median location; 2,422 to 3,647 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% | 419 kWh | 2,577 kWh | £419 | £215 |
| 19% | 569 kWh | 2,427 kWh | £440 | £248 |
| 23% | 689 kWh | 2,307 kWh | £457 | £275 |
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
- households wanting the largest system that avoids a network application
- roofs that take nine or ten panels comfortably
- anyone whose installer has proposed a 3.68 kW inverter
Roughly 9 panels at 430 W, needing about 18 m² of roof. Higher-rated modules need fewer panels for the same capacity, which matters on a tight roof.
Where the odd number comes from
G98 — the connect-then-notify process — applies up to and including 16 A per phase, which the Energy Networks Association’s own document converts to 3.68 kW on a single-phase supply.
That threshold has shaped the domestic market for years. It is why so many inverters are rated at exactly 3.68 kW, and why quotes cluster just below 4 kWp.
The limit is on the inverter’s registered capacity, not on panel capacity. A 5 kWp array behind a 3.68 kW inverter has a registered capacity of 3.68 kW and stays inside G98. That is a legitimate and common design: the inverter clips a handful of peak summer hours, and in exchange you avoid an application process.
What to ask: if the array is larger than the inverter, how much output is expected to be clipped, and what does that cost across a year? A good installer has modelled it.
Is avoiding G99 worth it?
For a domestic system, G99 means applying and waiting for a decision rather than notifying afterwards — see how to apply for a G99 connection. It is time and paperwork rather than a large fee, and the operator may impose export limitation.
If your roof genuinely supports 6 kWp and you will use the output, the process is worth going through. If it supports 4 kWp, staying under the threshold is the path of least resistance.
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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