Estimate the solar system size and number of panels you need.
Calculated instantly in your browser.
How do you calculate how many solar panels you need?
System size (kW) = daily usage ÷ (peak sun hours × efficiency), then Panels = system watts ÷ panel watts, rounded up. Peak sun hours are the equivalent hours of full-strength sunlight per day; the efficiency factor covers inverter, wiring and heat losses. For example, 30 kWh a day at 5 sun hours and 80% efficiency needs a 7.5 kW system, about 19 panels of 400 W.
Understanding your result
Peak sun hours are the equivalent hours of full-strength sunlight per day for your location. The efficiency factor covers losses from the inverter, wiring, dust and temperature, so the real array is larger than the raw demand.
Formula and method
System size (kW) = daily usage ÷ (peak sun hours × efficiency). Panels = round up of system watts ÷ panel watts.
Worked example
30 kWh a day with 5 peak sun hours at 80% efficiency needs a 7.5 kW system — about 19 panels of 400 W.
How to use this tool
- Enter your daily energy usage in kWh.
- Enter your local peak sun hours.
- Set the panel wattage and efficiency.
Common mistakes to avoid
- Using daylight hours instead of peak sun hours.
- Ignoring system losses by assuming 100% efficiency.
About the Solar Panel Calculator
The Solar Panel Calculator estimates the solar system size and number of panels needed to cover your energy use, based on your daily usage, local peak sun hours, panel wattage and system efficiency.
Who should use this tool
Homeowners and off-grid users planning a solar installation.
Benefits
- Recommended system size in kW.
- Number of panels for your wattage.
- Accounts for real-world system losses.
- Shows the working.
Practical use cases
- Sizing a rooftop solar system.
- Planning an off-grid or RV setup.
- Estimating panels before getting quotes.
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Frequently asked questions
What are peak sun hours?
The number of hours per day when sunlight averages 1,000 W/m² — typically 3 to 6, depending on location and season.
Why include an efficiency factor?
Real systems lose energy to the inverter, wiring, heat and dirt, so you need a larger array than the bare demand suggests.