The average three-bedroom semi-detached home in the UK consumes between 3,200 kWh and 4,500 kWh of electricity per year, according to Ofgem's 2025 household energy statistics. But annual consumption is only one input. You also need to account for how much usable solar energy your roof can generate — and that varies significantly across UK regions.
How UK Irradiance Affects Your System Size
The UK receives considerably less solar irradiance than southern Europe, but still generates meaningful power throughout the year. The key metric is peak sun hours (PSH) — the number of equivalent hours per day at 1,000 W/m² irradiance.
| Region | Annual PSH (south-facing, 35° tilt) | Expected yield per kWp |
|---|---|---|
| South-west England | 1,050–1,100 | 900–960 kWh |
| South-east England | 1,000–1,050 | 850–910 kWh |
| Midlands | 950–1,000 | 800–860 kWh |
| Yorkshire / North-west | 900–960 | 760–820 kWh |
| Scotland (central) | 850–920 | 720–780 kWh |
These figures assume a south-facing roof at 30–40°. East or west-facing roofs produce roughly 80% of the south-facing yield. North-facing roofs are not recommended.
Calculating the Right kWp for Your Home
The standard approach is to divide your annual consumption by the expected annual yield per kWp for your region.
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Example: 3-bed semi in the Midlands, annual consumption 3,800 kWh, no EV or heat pump
- Expected yield: 830 kWh per kWp
- Theoretical system size: 3,800 ÷ 830 = 4.58 kWp
- Practical recommendation: 4.0–5.0 kWp (8–10 x 400W panels)
But generating 100% of your consumption in theory does not mean consuming 100% of your solar output. Self-consumption rates without battery storage typically run at 30–45% for an occupied daytime home, rising to 50–65% with a battery.
Adjusting for EV Charging and Heat Pumps
A battery electric vehicle adding 3,000–4,000 kWh annually (typical for 8,000–12,000 miles) or an air-source heat pump adding 2,000–3,500 kWh will substantially change the calculation.
Revised example: same home plus an EV and heat pump
- Base consumption: 3,800 kWh
- EV addition: 3,500 kWh
- Heat pump addition: 2,500 kWh
- Total annual consumption: 9,800 kWh
- Theoretical system size (Midlands): 9,800 ÷ 830 = 11.8 kWp
Fitting 11.8 kWp on a standard 3-bed semi is challenging — a typical roof offers space for 8–14 panels per slope. In this scenario, a 6–8 kWp system with a 10–15 kWh battery is often more practical and cost-effective, supplemented by off-peak tariff charging (such as Octopus Intelligent Go at approximately 7.5p/kWh off-peak in 2026).
Panel Count and Roof Space Requirements
Modern 420W–440W monocrystalline panels measure approximately 1.75m x 1.13m.
- 4 kWp system: ~10 panels, ~20 m² of usable roof space
- 6 kWp system: ~14 panels, ~28 m²
- 8 kWp system: ~18–19 panels, ~37 m²
Allow for a 300–500mm border from eaves and roof edges, and account for any Velux windows, soil pipes, or chimney stacks.
Battery Sizing Guidance
For a 3-bed semi without significant daytime demand:
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- No EV or heat pump: 5–10 kWh battery (e.g. GivEnergy AIO 9.5, BYD Battery-Box Premium HVS 10.2)
- With EV (smart-charged overnight): 10–15 kWh (e.g. Tesla Powerwall 3 at 13.5 kWh)
- With heat pump: 10–15 kWh, prioritise systems with high charge/discharge rates
