Annual Usage: Monthly kWh × 12 Target Generation: Annual Usage × Offset Target Required kW: Target Generation / (365 × Peak Sun Hours × Performance Ratio) Panel Count: (Required kW × 1000) / Panel Wattage Roof Area: Panel Count × Typical Panel FootprintAnnual Usage: Monthly kWh × 12 Target Generation: Annual Usage × Offset Target Required kW: Target Generation / (365 × Peak Sun Hours × Performance Ratio) Panel Count: (Required kW × 1000) / Panel Wattage Roof Area: Panel Count × Typical Panel Footprint.