Solar Array Sizing Calculator
Calculate how many solar panels you need to meet your daily energy load.
Estimates are for system planning only. Add a 10–25% margin for cloudy days and future load growth. Verify sizing with a qualified solar installer and local electrical codes.
How it works
- 1Enter your daily energy need in kWh, the panel wattage, and your local peak sun hours.
- 2Set the system derate % (typically 75–80%) for wiring, heat, and inverter losses.
- 3The calculator divides your load by each panel’s daily output and rounds up to the panel count.
Use cases
- Sizing a rooftop system before getting installer quotes.
- Comparing how a panel-wattage upgrade reduces the count needed.
- Estimating array capacity for an off-grid cabin or RV.
Frequently asked questions
How is the panel count calculated?
Each panel’s daily output = (W × sun hours × derate) ÷ 1000. A 400 W panel at 5 sun hours and 75% derate makes 1.5 kWh/day, so 10 kWh ÷ 1.5 rounds up to 7 panels.
What is the system derate factor?
Derate accounts for heat, wiring, inverter efficiency, soiling, and mismatch. 75% (0.75) is the common default; high-quality systems reach 80–85%.
Should I add extra panels?
Yes — add a 10–25% margin for cloudy days, seasonal variation, and future load growth. A qualified installer can assess your site.
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