Solar PV Sizing Calculator — UK

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.

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.

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.

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. Use our Electrical Load Calculator to understand your consumption and how much solar can offset.

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.