Free Online Solar Panel Calculator to estimate photovoltaic system size, energy production, and cost savings based on your location, roof area, and energy consumption. Accurate area estimation ensures optimal panel placement, maximizes energy harvest, and prevents shading or structural conflicts. If you're consuming 1,000 kWh per month in a sunny state like California, you might need just 16 panels, while the same. . A typical home solar panel is about 3 feet wide by 5. 5 feet long, occupying an area of roughly 17. Formula: Panels = (Roof Area × Usable % × (1 − Spacing Loss %)) ÷ Panel Area → Total Capacity (kW) = Panels × Panel Wattage ÷ 1000. Determining how many solar panels fit on. .