Solar Panel System Size Calculator

System size is calculated by working backward from your annual electricity consumption. We start with your monthly kWh usage (or estimate it from your bill and rate), multiply by 12 for annual usage, then apply your desired offset percentage (e.g., 100% for full coverage, 80% for partial offset).

That target annual production is divided by your state's average daily peak sun hours times 365 days, with a 14% system loss factor applied. The result is the system size in kW.

Panel count is calculated using your selected panel wattage (default 400W, the current residential standard). Required roof area is approximately 17.5 sqft per panel — a typical figure for modern 60-cell panels.

Sizing a solar system is fundamentally about matching annual production to annual consumption. The right size depends on three factors: your home's electricity usage, your location's solar resource (peak sun hours), and what percentage of your usage you want to offset.

Modern residential systems range from about 4 kW (small home, partial offset) to 15 kW (larger home or planned EV charging). The U.S. average residential install is around 8 kW. Production capacity in kWh per year roughly equals system size × sun hours × 365 × 0.86 (after losses).

Panel count follows directly from system size and the wattage of the panels you choose. Most installers in 2026 use 400–450W panels. A typical 8 kW system uses 20 panels, occupies about 350 sqft of unshaded roof, and produces 11,000–13,000 kWh per year in a sunny climate. If your roof is smaller than ideal, higher-efficiency panels (430W+) let you fit more capacity into less space — at a slightly higher cost per watt.