Solar Panel Calculator
Calculate how many solar panels you need, the system size in kWp and your estimated monthly generation based on your electricity consumption and location.
What this calculates
Calculates the required system size in kWp, number of panels, actual system size, estimated monthly generation, self-sufficiency percentage and roof area needed.
Formula used
Daily consumption = monthly ÷ 30. kWp needed = daily ÷ (PSH × 0.80). Panels = ceil(kWp × 1000 ÷ wattage). Generation = kWp × PSH × 0.80 × 30 days.
Worked example
Monthly use: 350 kWh. PSH: 5 h/day. Panel: 400 W. Daily: 11.67 kWh. kWp needed: 2.92. Panels: 8 (actual: 3.2 kWp). Monthly generation: ≈ 384 kWh. Self-sufficiency: ~100%.
Peak sun hours by region
Sunny climates (Australia, Middle East, SW USA, N. Africa): 5.5–7 h/day. Mediterranean / S. Europe: 4.5–5.5 h/day. Central Europe / UK: 2.5–3.5 h/day. Look up your specific location in a solar resource map (e.g., Global Solar Atlas) for an accurate figure.
When not to use this calculator
This calculator assumes the system is grid-connected and sized to cover average monthly consumption. Off-grid systems, battery storage sizing and shading analysis require a detailed solar design study.
Frequently asked questions
How many solar panels do I need?
Daily kWh ÷ (peak sun hours × 0.80) = kWp needed. Divide by panel wattage and round up. The calculator does this automatically.
What are peak sun hours?
Equivalent daily hours of full sun (1,000 W/m²). Typically 2.5–7 depending on location. Find yours on Global Solar Atlas.
What is kWp?
Kilowatt-peak — the rated output of your solar system. 10 panels × 400 W = 4 kWp system.
How much roof space do I need?
About 2 m² per panel. A 10-panel system needs roughly 20 m² of unshaded roof.
Assumptions
- System efficiency: 80% (accounts for inverter, wiring and temperature losses).
- Roof area: 2.0 m² per panel (standard for modern 400 W panels).
- Self-sufficiency capped at 100% — surplus generation not shown.
- Does not include battery storage, shading factors or orientation losses.
Actual generation varies with roof orientation, shading, local climate and system losses. A professional solar design using satellite irradiance data (e.g., PVsyst) is recommended before purchasing equipment.
Common mistakes
- Using average daily sun hours instead of peak sun hours — these are different values.
- Not accounting for shading from trees, chimneys or adjacent buildings.
- Sizing only for current consumption without considering future EV charging or heat pump loads.