Solar Panel Calculator: How Many Panels Do You Actually Need?
Enter your monthly electricity usage and your state. We compute the exact number of 400W panels required to offset your usage, the system size in kilowatts, and the roof area you need to fit them.
How Panel Count Is Calculated
Panel count starts from your monthly electricity usage in kilowatt-hours. We divide by 30 to get a daily usage figure, then divide that by your state's peak sun hours per day adjusted by an industry-standard 80% derate factor (which accounts for inverter losses, soiling, wiring losses, and temperature derate).
That gives the kilowatt rating your system needs. Dividing by 0.4 kW (the wattage of a typical residential panel in 2026) gives the panel count, rounded up. Roof space requirement assumes 17.5 sq ft per panel including spacing.
Real systems are often slightly oversized to account for future EV charging, heat pump conversion, or general electrification. If you're planning to electrify, model 30-50% above current kWh usage.