Solar System Size Calculator
Find out exactly how many solar panels you need and what system size fits your home.
How Solar System Sizing Works
The Sizing Formula
The system size is calculated by dividing your daily energy need (monthly kWh ÷ 30) by peak sun hours and system efficiency. This gives you the DC system size in kilowatts. Panel count is then determined by dividing system watts by each panel's wattage rating.
Peak Sun Hours
Peak sun hours represent the number of hours per day when sunlight intensity averages 1,000 W/m². This varies significantly by location — the US Southwest averages 5–6 hours, the Midwest 4–4.5 hours, and the Northeast 3.5–4 hours. Use your local value for the most accurate sizing.
System Efficiency
A real solar installation loses energy to heat, wiring resistance, inverter conversion, and panel mismatch. A realistic efficiency factor is 75–85%. Using 80% is a common and conservative estimate that accounts for real-world losses.
Roof Area
A standard 400W solar panel is approximately 17.5 square feet (about 6.5 ft × 2.7 ft). Panels are usually installed with some spacing, so plan for 20–25 sq ft per panel including mounting clearance. Roof pitch, shading, and orientation also affect usable area.
Offset Percentage
Installing at 100% offset means your system is designed to produce exactly as much as you use annually. Many homeowners choose 90% to reduce cost, while others choose 110% to account for future EV charging or increased usage.