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Solar Panel Sizing Calculator — Domestic PV System Design
Calculate the ideal solar PV system size for your home based on electricity usage, roof orientation, pitch, and location in the UK.
Check your energy bills for annual total
Angle from horizontal (30-40° optimal)
Usable area excluding obstructions
Typical modern panels are 380-430W
Most 400W panels are ~1.7–2.0 m² (≈1.9 m²)
Ofgem cap is 26.11p/kWh (Jul–Sep 2026) — use your own tariff
Smart Export Guarantee rate
Typical 30-50% without battery
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How We Calculate This
This calculator estimates the optimal solar PV system size for your property based on your annual electricity consumption and roof characteristics. It uses UK-specific solar irradiance data adjusted for orientation, pitch, and regional location.
Calculation methodology
- Base irradiance: UK regional specific yield ranges from about 850 kWh/kWp/year in Scotland to 1,050 kWh/kWp/year in southern England (consistent with MCS/SAP location data)
- Orientation factor: Adjusts output based on roof compass direction (south=100%, east/west=80%, north=55%)
- Pitch factor: Optimal tilt is 30-40 degrees for UK latitude; steeper or flatter roofs lose some output
- System sizing: Divides annual usage by effective irradiance to find the ideal system size in kWp
Financial returns
Savings come from two sources: avoiding grid electricity purchases (self-consumption) and income from exporting surplus to the grid under the Smart Export Guarantee. Self-consumed electricity saves at the full retail tariff rate, making higher self-consumption the key to better returns.
Frequently Asked Questions
A typical UK household uses around 3,500-4,000 kWh per year. With a south-facing roof in southern England, a 4 kWp system (10 x 400W panels) would generate roughly 3,800-4,200 kWh annually. Homes with higher usage or less optimal orientations may need more panels.
Due south is optimal, generating the maximum annual output. South-east and south-west orientations lose only about 5% output. East or west-facing roofs lose around 20%. North-facing roofs are generally not recommended as they lose 40-50% of potential output.
Self-consumption is the percentage of solar generation you use directly rather than exporting. Since you save the full electricity rate (the Ofgem cap is 26.11p/kWh for July-September 2026 — check the current cap and your own tariff) on self-consumed energy but only receive the export tariff (typically around 5.5p/kWh, though some SEG tariffs pay more) for exported energy, higher self-consumption significantly improves financial returns. Battery storage and smart scheduling can increase self-consumption from a typical 30-50% to 70-90%.
A standard 400W panel is approximately 1.7-2.0 m² (around 1.9 m², roughly 1.1m x 1.75m) depending on the model. A typical 4 kWp system of 10 panels needs around 19 m² of usable roof space. You also need to account for obstructions like chimneys, roof windows, and ventilation pipes.
The Smart Export Guarantee requires licensed electricity suppliers with 150,000+ customers to offer a tariff for exported solar electricity. Rates vary by supplier and tariff type, typically ranging from 3p to 15p per kWh. Some suppliers offer time-of-use export rates paying more during peak demand periods.
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Last updated: June 2026
Verified against UK standards · estimates only, confirm with your supplier.