Solar Panel Calculator
Calculate how many solar panels you need to offset your electricity usage. Enter your monthly electric bill or kWh usage, choose your region's sun hours, and get system size, panel count, roof space requirements, and estimated annual savings.
Enter Your Energy Details
Full Breakdown
Annual Usage
Target Offset
System Size
Panels Needed
Roof Space
Annual Production
Est. Annual Savings
Est. System Cost
How to Use This Calculator
- Enter your monthly kWh usage (check your electric bill) or your monthly bill amount.
- Select your area's peak sun hours — this is the average daily equivalent hours of full sunlight.
- Choose the panel wattage — 400W is the current residential standard.
- Set your offset target — 100% means fully offsetting your usage.
- Click "Calculate Solar System" for your results.
The Formula
Daily kWh Needed = (Monthly kWh × Offset%) ÷ 30
System Size (kW) = Daily kWh ÷ Peak Sun Hours ÷ 0.80
Number of Panels = System kW × 1000 ÷ Panel Wattage
The 0.80 factor accounts for real-world system losses (inverter efficiency, wiring, temperature, shading, soiling). Typical losses range 15–25%. Each panel needs approximately 17.5 sq ft of roof space (standard 65" × 39" panel).