How Many Solar Panels Do I Need? UK Sizing Guide

Step 1: Check your electricity usage

Look at your electricity bills or smart meter data for your annual consumption in kWh. The average UK household uses around 2,900 kWh per year, but this varies significantly. If you have an electric vehicle or heat pump, your usage will be much higher — potentially 5,000–8,000 kWh per year.

Step 2: Match to a system size

As a rule of thumb, aim for a solar system that generates roughly 80–100% of your annual electricity usage. In central England, each kWp of solar generates approximately 850–950 kWh per year on a south-facing roof. So if you use 2,900 kWh per year, a 3–3.5kW system would roughly match your consumption.

However, because you cannot use all your solar electricity directly (some is generated while you are out), many homeowners opt for a slightly larger system to maximise savings — especially if they plan to add a battery or an EV charger later.

Step 3: Check your roof space

Each standard 400W panel measures approximately 1.7m x 1.0m (1.7m²). You need roughly:

• 8 panels (3kW): ~14m² of unshaded roof
• 10 panels (4kW): ~17m² of unshaded roof
• 13 panels (5kW): ~22m² of unshaded roof
• 15 panels (6kW): ~26m² of unshaded roof

This is the usable area — subtract space for roof obstructions like skylights, vents, chimneys, and areas with significant shading. Your installer will calculate the exact layout during a site survey.

Step 4: Factor in roof direction

Roof direction affects how much each panel generates. South-facing is optimal. If your roof faces east or west, each panel generates about 20% less, so you may need more panels to achieve the same output:

• South: 100% output — 10 panels for 4kW
• South-east / south-west: 95% output — 10–11 panels for effective 4kW
• East / west: 80% output — 12–13 panels for effective 4kW

1–2 bed flat or terrace

Typical usage: 1,800–2,400 kWh/year. A 2–3kW system (5–8 panels) is usually sufficient. Roof space on terraced houses can be limited, so check with an installer whether there is enough unshaded area for the panels you need. Flats may require freeholder consent.

3-bed semi-detached or detached

Typical usage: 2,900 kWh/year. A 4kW system (10 panels) is the most popular choice. Most semi-detached and detached homes have enough roof space on one face. If you work from home or plan to get an EV, consider stepping up to 5kW (13 panels).

4–5 bed detached

Typical usage: 3,800–4,500 kWh/year. A 5–6kW system (13–15 panels) matches higher consumption. Larger homes usually have ample roof space. Households with electric vehicles or heat pumps should consider the upper end or even larger systems if roof space allows.