Calculator
Solar Panel Sizing Calculator
Get a precise solar panel count for your roof in under 2 minutes
Figuring out how many solar panels your home needs doesn't have to involve guesswork. This calculator uses your monthly electricity consumption, available roof space, and local solar conditions to determine the optimal number and size of solar panels for your property. Whether you're planning a DIY installation or getting quotes from contractors, accurate sizing ensures you don't overpay for capacity you won't use—or underestimate what you actually need.
Enter Your Details
Fill in the form and click Calculate to see results.
Overview
Figuring out how many solar panels your home needs doesn't have to involve guesswork. This calculator uses your monthly electricity consumption, available roof space, and local solar conditions to determine the optimal number and size of solar panels for your property. Whether you're planning a DIY installation or getting quotes from contractors, accurate sizing ensures you don't overpay for capacity you won't use—or underestimate what you actually need.
Use this result
Use the calculator inputs first, then compare the result against local rates, incentives, roof conditions, and utility export rules.
Method, assumptions, and sourcesOpen this section when you want to audit the calculation behind the estimate.ShowHide
Calculation Method
The calculator determines system size by dividing your annual energy requirement by the expected solar production per panel. We use the standard formula: System Size (kW) = Annual Energy (kWh) / (Peak Sun Hours × 365 × System Efficiency). Panel count is then derived by dividing system size by individual panel wattage, with a derating factor of 0.85 to account for real-world losses from inverter efficiency, wiring, temperature, and soiling—aligned with NREL PVWatts v8 recommendations. Peak sun hours are sourced from the National Solar Radiation Database for your specific US location. The tool also checks whether your available roof area can physically accommodate the calculated panel count, assuming standard 17.5 sq ft per 400W panel.
Key Assumptions
- Solar panel count depends on annual kWh usage, usable roof area, local sun hours, inverter losses, and panel wattage.
- Production estimates assume an unshaded roof section and typical residential system losses.
- Final system sizing should be checked against installer layout, utility interconnection, and local code requirements.
Formula Assumptions Data Sources FAQ Related Links
How to Use This Calculator
Start by entering your average monthly electricity bill or kilowatt-hour usage—you can find this on your utility statement. Next, select your location from the US region dropdown, which adjusts solar irradiance data automatically. Enter your available roof space in square feet (measure the unshaded, south-facing portion). The calculator instantly shows how many panels you need, the recommended system size in kilowatts, estimated annual production, and whether your roof has sufficient area. Adjust panel wattage if you have a specific product in mind (400W is the default, reflecting current market standards).
Formula & Methodology
The calculator determines system size by dividing your annual energy requirement by the expected solar production per panel. We use the standard formula: System Size (kW) = Annual Energy (kWh) / (Peak Sun Hours × 365 × System Efficiency). Panel count is then derived by dividing system size by individual panel wattage, with a derating factor of 0.85 to account for real-world losses from inverter efficiency, wiring, temperature, and soiling—aligned with NREL PVWatts v8 recommendations. Peak sun hours are sourced from the National Solar Radiation Database for your specific US location. The tool also checks whether your available roof area can physically accommodate the calculated panel count, assuming standard 17.5 sq ft per 400W panel.