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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
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
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Last updated: June 2026
Verified against UK standards · estimates only, confirm with your supplier.