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Solar PV Calculator

Estimate system capacity, annual generation, energy bill savings, SEG export earnings, payback period, and CO2 offset for a rooftop solar PV installation.

Solar PV Calculator

System size, annual yield, savings, SEG earnings, payback & CO2

Size from roof area
Enter available roof space - calculator works out how many panels fit and the system output
Enter panel count
Know how many panels you want or have been quoted - enter the number directly
🇬🇧 UK & Ireland
🇺🇸 USA & Canada
🇪🇺 Europe
🇦🇺 Aus & NZ
🌍 Other
★ 850 kWh/m²/yr
Flat (0°)
15°
25°
35°
45°
60°
★ Yield factor: 1.00 (optimal)
400 W
450 W
500 W
550 W
■ ~1.93 m² per panel
Measure the usable roof plane - exclude ridges, hips, verges, and Velux windows.
Chimney stacks, roof lights, vents. Leave blank if none.
■ Approx. 0 panels fit

Tariffs & Usage

Unit rate from your energy bill in pence, cents, or eurocents per kWh
UK: Smart Export Guarantee (SEG). US: net metering credit. AUS: feed-in tariff.
Typical household: 30-40%. Higher with battery storage.
Full installed cost in local currency. Used to calculate payback period.

Solar PV Results

Global Irradiance Guide

LocationkWh/m²/yr
UK & Ireland
Scotland / N. Ireland~850
Midlands / Wales~1,000
South England / SW~1,100-1,150
USA & Canada
Pacific NW / Alaska~1,000-1,200
NE / Midwest / Canada~1,300-1,450
SE / Florida / Texas~1,500-1,700
Southwest / Sun Belt~1,800-2,100
Europe
Scandinavia~900-1,000
Central Europe~1,000-1,200
Mediterranean~1,500-1,900
Australia & NZ
South / New Zealand~1,400-1,600
SE Australia~1,600-1,800
QLD / WA / NT~1,900-2,300

For precise site data: PVGIS (Europe/global), PVWatts (USA), Solar Atlas (worldwide).

Orientation & Pitch

OrientationFactor
South1.00
SE / SW0.96
East / West0.85
NE / NW0.65
North0.52

Northern hemisphere: south-facing is optimal. Southern hemisphere (Aus/NZ): north-facing is optimal - the calculator adjusts automatically. East/West split arrays can work well; calculate each roof plane separately.

Panel Size Guide

WattageApprox. size
400 W1,722 × 1,040 mm
450 W1,855 × 1,040 mm
500 W2,094 × 1,048 mm
550 W2,278 × 1,040 mm

Higher wattage panels generate more power per m² - fewer panels needed for the same output. Check manufacturer data sheets for exact dimensions.

Battery Storage

  • Without battery: typical self-consumption is 30-40% of generated electricity.
  • Adding a battery can raise self-consumption to 60-80% by storing daytime surplus for evening use.
  • Popular sizes: 9.5 kWh (GivEnergy, SolarEdge), 13.5 kWh (Tesla Powerwall 2), 5 kWh (entry level).
  • Battery payback is typically longer than panels alone - factor in battery replacement (10-15 yr life).

Key Terms

  • kWp - kilowatt peak: rated output at standard test conditions (1000 W/m², 25°C).
  • Performance Ratio (PR) - real-world losses from inverter, wiring, temperature, soiling. Typically 0.80-0.85.
  • Export / feed-in rate - UK: Smart Export Guarantee (SEG). USA: net metering credit rate. Australia: feed-in tariff (FiT). Enter in pence, cents, or eurocents per kWh.
  • Self-consumption - the share of generated electricity used in the building (not exported).