How many solar panels can I fit on my roof?

A standard residential solar panel is approximately 1.7m × 1.0m (1.7m²) and produces 400-425W. A typical UK semi-detached house has about 20-25m² of usable south-facing roof, fitting 12-15 panels (4.8-6.0kWp). Allow for chimneys, vents, Velux windows, and minimum 300mm edge clearance. Panels should not extend beyond the roof line, and scaffold access may be needed.

How much electricity will solar panels generate in the UK?

In the UK, a well-oriented (south-facing, 30-35° pitch) 4kWp system generates approximately 3,400-4,200kWh per year depending on region: 1,050kWh/kWp in south England, dropping to 800kWh/kWp in northern Scotland. A 4kWp system in the Midlands (950kWh/kWp) would generate about 3,800kWh — roughly the annual consumption of a typical UK household. Output varies seasonally: ~80% is generated between April and September.

Does roof orientation affect solar panel output significantly?

Yes — due south at 30-35° pitch is optimal (100%). South-east or south-west facing loses only about 5%. East or west facing loses about 15%. North-facing roofs lose 40-50% and are generally not recommended. Flat roofs (0-10°) achieve about 89% of optimum but can be tilted using mounting frames. A well-designed east/west split across a dual-pitch roof can still be highly effective, with better spread of generation throughout the day.

What size inverter do I need for my solar panels?

The inverter converts DC from the panels to AC for your home. It should be sized at 80-100% of the array kWp — slightly undersizing (e.g., 3.6kW inverter for 4kWp array) is common because panels rarely achieve nameplate power in UK conditions. String inverters are cheaper but affected by partial shading. Microinverters or power optimisers (one per panel) cost more but maximise output when panels are partially shaded or on multiple orientations.

Do I need planning permission for solar panels in the UK?

In England and Wales, rooftop solar panels on a house are usually Permitted Development — no planning permission needed — provided: panels don't protrude more than 200mm from the roof plane, don't extend above the highest point of the roof, and are not on a wall facing a highway. Listed buildings and conservation areas DO need planning permission. Scotland and Northern Ireland have similar but not identical rules. Always check with your local planning authority.

TradeCalculator

Solar PV Sizing Calculator — UK

Size a solar PV system from your available roof area, orientation, and UK region. Calculate panel count, system kWp, annual yield, inverter size, and estimated savings.

Usable Roof Area

Available area for panels

Roof Pitch

Angle from horizontal (35° optimal)

Roof Orientation

Direction the roof faces

UK Region

Determines solar irradiation

Panel Wattage

Nameplate power per panel (400W typical)

Annual Electricity Usage

From your electricity bill

How We Calculate This

This calculator estimates solar PV system size and annual energy yield based on your roof area, orientation, pitch, and UK regional solar irradiation data.

Calculation Method

Annual yield = kWp × Regional yield × Orientation factor × (1 - Losses) × Shading factor

UK Regional Yields (kWh/kWp/year)

South England: 1,050 kWh/kWp
Midlands: 950 kWh/kWp
North England: 880 kWh/kWp
Scotland (south): 850 kWh/kWp
Scotland (north): 800 kWh/kWp
System losses: ~14% (inverter, wiring, temperature, dirt)

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.