Solar System Size Calculator
Calculate how many solar panels you need for your home.
Enter your electricity usage to find the right system size in kW.
To size a solar system, you need to match your annual electricity consumption against the expected output from your location.
Sizing formula: System size (kW) = annual consumption (kWh) / (365 × peak sun hours × system efficiency)
Typical household consumption:
- Small flat / apartment: 1,500–2,500 kWh/year
- Small house (2 bedroom): 2,500–3,500 kWh/year
- Medium house (3-4 bedroom): 3,500–5,000 kWh/year
- Large house or EV charging: 5,000–10,000 kWh/year
Worked example: Home uses 4,200 kWh/year, located in the UK (3.5 peak sun hours), 85% system efficiency: System size = 4,200 / (365 × 3.5 × 0.85) = 4,200 / 1,088 = 3.86 kW
That is approximately 10 × 400W panels.
Roof space required:
- A standard 400W panel: ~1.7 m × 1.0 m = 1.7 m²
- A 4 kW system (10 panels): ~17–22 m² (accounting for spacing and walkways)
- South-facing roofs in the northern hemisphere produce the most power; SE/SW within 20° of south is nearly as good.
Battery sizing: A solar battery (5–15 kWh capacity) increases self-consumption from ~30% to 70%+ but adds £3,000–£8,000 to system cost. Most worthwhile for homes with high evening electricity use or EV charging.