How Many Solar Panels Do I Need?

The number of solar panels you need depends on your annual electricity usage, your roof space, and how much of your usage you want to cover with solar.

The Quick Formula

To work out how many panels you need:

1. Find your annual electricity usage in kWh (check a recent bill or annual statement)
2. Divide by 350 (typical kWh per panel per year in the UK)
3. Round up to the nearest whole number

For example, if you use 3,500 kWh/year, you need roughly 10 panels (3,500 ÷ 350 = 10).

UK Average Electricity Usage

Most UK homes use between 2,000 and 5,000 kWh/year depending on house size, occupancy, and heating type:

1-2 bedroom flat

2,000-2,500 kWh/year → 6-7 panels

3 bedroom house

2,900 kWh/year (UK average) → 8-9 panels

4 bedroom house

3,500-4,500 kWh/year → 10-13 panels

5+ bedroom house

5,000+ kWh/year → 14-16 panels

If you have an electric vehicle or heat pump, add 2,000-3,000 kWh to these figures (roughly 6-9 extra panels).

Panel Wattage Matters

Modern solar panels range from 350W to 450W. Higher-wattage panels generate more electricity per panel, so you need fewer of them:

• 350W panel: ~300-350 kWh/year in the UK
• 400W panel: ~350-400 kWh/year
• 450W panel: ~400-450 kWh/year

A 4 kW system could be 10× 400W panels or 9× 450W panels. Higher-wattage panels cost slightly more but take up less roof space per kW.

Roof Space Requirements

Each panel occupies roughly 1.7-2.0 square metres depending on wattage. A 10-panel system needs 17-20m² of usable roof space.

Not all of your roof is usable. Subtract space for:

• Chimneys, skylights, and vents
• Edge clearances (typically 30cm from roof edges)
• Shaded areas (trees, neighbouring buildings)

If your usable roof space is smaller than your electricity usage suggests, you have three options:

1. Install a smaller system and cover less of your usage
2. Use higher-wattage panels (450W instead of 400W)
3. Install panels on multiple roof faces (e.g. south and west)

System Size vs Coverage

You don't need to cover 100% of your electricity usage. In fact, most UK homes can't because of roof constraints and seasonal mismatch (solar generates most in summer when usage is lowest).

Typical coverage rates:

• 40-50% coverage: Small system (2-3 kW), cheaper but lower savings
• 60-80% coverage: Medium system (4-5 kW), typical for UK homes
• 90%+ coverage: Large system (6-10 kW), requires battery storage to avoid wasting summer surplus

Without a battery, you'll export 50-70% of what you generate (because you're not home during peak generation). The Smart Export Guarantee (SEG) pays you for exports, but at ~5.5p/kWh vs 24p/kWh you'd pay for grid electricity, so self-consumption is more valuable than export.

Seasonal Variation

Solar panels in the UK generate roughly 3× more in summer than winter:

• June-August: 40-45% of annual generation
• December-February: 10-15% of annual generation

This means even a system sized to cover 70% of your annual usage will only cover 20-30% in winter (when you use most electricity for heating and lighting). A battery helps shift summer surplus to winter evenings, but can't solve the seasonal mismatch entirely.

Example: 4 kW System for a 4-Bed House

Annual usage: 3,500 kWh
System size: 4 kW (10× 400W panels)
Annual generation: 3,800 kWh (Midlands irradiance)
Self-consumed: 1,520 kWh (40% without battery)
Exported: 2,280 kWh
Coverage: 43% of annual usage

With a battery (increasing self-consumption to 70%), coverage rises to 76% of annual usage.