Solar PV Panel Sizing Calculator
Estimate the solar system size and number of panels needed for your home.
How this PV panel calculator works
This PV panel calculator estimates the solar array size you need based on your daily energy use, local sun exposure, and panel specifications. It is designed for quick planning and early budgeting before speaking with an installer.
The tool uses practical assumptions and applies a system efficiency factor to account for real-world losses such as inverter conversion, cable losses, dirt on panels, temperature impact, and shading.
Formula used in the calculator
1) Required solar system size
Required system size (kW) = Daily usage (kWh) ÷ (Peak sun hours × Efficiency)
2) Number of panels
Number of panels = Ceiling of [(Required system size × 1000) ÷ Panel wattage]
3) Roof space needed
Roof area needed (m²) = Number of panels × Area per panel
Example calculation
If your home uses 20 kWh/day, your site gets 5 sun hours/day, your panels are 400W, and your system efficiency is 80%:
- Required system size = 20 ÷ (5 × 0.8) = 5 kW
- Estimated panels = (5000 ÷ 400) = 12.5, rounded up to 13 panels
- If each panel is 2.0 m², roof space needed ≈ 26 m²
Tips for better accuracy
- Use your annual utility bill to estimate true average daily consumption.
- Check local peak sun hour data by city or region.
- Use realistic efficiency values (typically 75% to 85%).
- Account for future load growth (EV charging, heat pump, pool pump).
- Include shading analysis and roof orientation in final design.
What affects PV performance?
Sunlight and climate
Higher irradiance and more clear days improve output, while cloudy or snowy conditions reduce generation.
Roof direction and tilt
South-facing roofs (in the northern hemisphere) often produce the highest annual output, with tilt optimized by latitude.
Temperature
Panels are less efficient when very hot, so real production can be lower than nameplate ratings during summer heat.
Final note
This calculator gives a strong starting point for system sizing. For installation decisions, always confirm with a certified solar professional who can perform a site survey, shade study, and electrical design.