Solar Panel Calculator
Estimate a preliminary solar panel count from your monthly electricity use, average peak sun hours, panel rating, and system-performance allowance.
How this calculator works
Monthly use should come from a recent utility bill and represent the consumption you want the system to offset. Peak sun hours are a location- and season-dependent average of equivalent full-sun hours, not simply daylight hours. Use a location-specific solar resource estimate for planning rather than assuming the example value applies to your roof.
daily energy = monthly use ÷ 30 estimated panels = ceil(daily energy ÷ (peak sun hours × performance factor × panel kW)) array size (kW DC) = panels × panel watts ÷ 1,000
Worked example
For 900 kWh per month, 5 peak sun hours per day, 400 W panels, and an 80% performance factor: average use is 30 kWh/day; 30 ÷ (5 × 0.80 × 0.4) = 18.75, so the preliminary estimate is 19 panels.
Performance factor is an assumption
The performance factor is an editable allowance for losses between panel rating and usable energy, including inverter and system effects. Actual production also depends on orientation, tilt, shading, temperature, equipment, and seasonal weather. The estimate is a simplified energy balance and does not model hourly production or utility billing rules.
A panel count is not a system design
This calculator does not assess roof structure, roof area, fire setbacks, electrical service, storage, interconnection, or permitting. A qualified solar installer should create a site-specific design and production estimate. Do not use this output to purchase or connect electrical equipment without professional review.
Frequently asked questions
How many solar panels do I need for my home?
A preliminary estimate depends on the energy use you want to offset, local solar resource, panel wattage, and system losses. A site-specific design must also consider roof layout, shading, equipment, and utility rules.
What are peak sun hours?
Peak sun hours represent the equivalent number of hours per day at a standard solar irradiance level. They are a way to summarize solar resource, not the total number of daylight hours.
What system performance factor should I enter?
It is a planning assumption that accounts for system losses. The appropriate factor depends on location, design, equipment, and site conditions; use a qualified installer’s estimate where possible.
Does this calculate battery size or savings?
No. It estimates panel count and approximate DC array size only. It does not model battery storage, hourly load, tariffs, incentives, shading, or financial payback.