Panel count is a capacity calculation, but a useful design also checks energy production, roof area, orientation, shading, inverter limits, and local rules.
The panel-count formula
Panel count = target system size in watts ÷ panel wattage. Round up because a fractional panel cannot be installed. A 5 kW target with 450 W panels is 5,000 ÷ 450 = 11.11, so the simple count is 12 panels.
Capacity is not annual production
A 5 kW array does not produce 5 kWh every hour. Sun hours, temperature, orientation, shading, inverter losses, soiling, and seasonal weather affect output. Use a generation estimate after sizing the nameplate capacity.
Roof and electrical constraints
Panel dimensions determine roof coverage, while string voltage and inverter limits constrain electrical layout. Leave access paths and account for setbacks or structural limitations. A count that fits on paper may not fit safely on the roof.
Use the result as a first pass
Get a site-specific proposal before purchasing equipment. Compare production assumptions, equipment warranties, degradation, interconnection requirements, and the treatment of export credits.
Sources
The figures and rules in this guide are checked against:
- Solar Photovoltaic System Design Basics — U.S. Department of Energy
- Solar Radiation Basics — U.S. Department of Energy
- Solar Photovoltaic Cell Performance and Efficiency Basics — U.S. Department of Energy
- Solar Explained: Photovoltaics and Electricity — U.S. Energy Information Administration