Solar PV Sizing Calculator (kW)
Estimate residential PV array size and annual production from usable roof area, peak sun hours, module specs, and a performance derate factor.
- Enter usable roof area Input the Usable roof area in square feet, excluding setbacks, vents, and shaded or obstructed sections.
- Set peak sun hours Enter the Peak sun hours per day for your location (default 4.5); use a local solar resource estimate for accuracy.
- Enter module specs Set the Module wattage in watts (default 400 W) and the Module area in square feet (default 21) for the panels you plan to use.
- Apply the performance factor and calculate Set the Performance factor (default 0.8 for inverter, wiring, soiling, and temperature losses) and press Calculate to get panel count, system size in kW, and annual kWh.
How many solar panels fit on a 600 sq ft roof?
The calculator divides usable roof area by module area, so at the default 21 sq ft per panel a 600 sq ft roof fits about 28 modules (it rounds down to whole panels). At 400 W each that is roughly an 11.2 kW system. Remember to enter usable area only — exclude setbacks, vents, and obstructions, which leave far less room than the raw roof footprint.
How is annual solar production estimated?
It follows the NREL PVWatts approach: daily kWh equals system size in kW times peak sun hours times the performance derate factor, then multiplied by 365 for the year. The default 0.8 derate accounts for inverter, wiring, soiling, and temperature losses. Peak sun hours vary by location and season, so use a local solar resource value rather than the 4.5 default for a real estimate.
What is the performance factor in solar sizing?
The performance factor (or derate) is a multiplier between 0.5 and 1 that captures real-world losses from the inverter, wiring, soiling, and high-temperature operation. A value of 0.8 is a common default, meaning the array delivers about 80 percent of its nameplate potential. Lower it for hot, dusty, or shaded sites and raise it for ideal conditions.