Solar Panel System Size Calculator
Sizing a rooftop solar photovoltaic system starts with your annual electricity consumption in kilowatt-hours and your location's peak sun hours per day. The standard formula from NREL and DOE energy calculators is: system size (kW) = annual kWh consumption divided by peak sun hours per day times 365 days times system efficiency (performance ratio). This calculator returns the required system size in kilowatts, the approximate number of panels at a given wattage, and the estimated annual generation to verify the offset percentage.
Solar system sizing formula
System kW = Annual kWh ÷ (Peak sun hours/day × 365 × system efficiency)
Panels = System kW × 1,000 ÷ panel wattage
Annual generation (kWh) = System kW × Peak sun hours/day × 365 × efficiency
Peak sun hours per day is the key site-specific variable. Use NREL's PVWatts calculator (pvwatts.nrel.gov) to find the precise value for your location and roof orientation. System efficiency of 0.77 (77%) is the DOE default derate factor, accounting for inverter efficiency, wiring, soiling, shading, and temperature losses.
Peak sun hours by US region (typical values)
- Southwest (Phoenix, Las Vegas, Los Angeles): 5.5 to 6.5 peak sun hours/day.
- Mountain West (Denver, Salt Lake City): 5.0 to 5.5 peak sun hours/day.
- Mid-Atlantic and Southeast (Atlanta, Washington DC): 4.5 to 5.0 peak sun hours/day.
- Midwest and Northeast (Chicago, New York, Boston): 4.0 to 4.7 peak sun hours/day.
- Pacific Northwest (Seattle, Portland): 3.5 to 4.0 peak sun hours/day.
- Use NREL PVWatts for precise local values including roof tilt and azimuth adjustments.
Solar system sizing: frequently asked questions
How many solar panels do I need for my home?
First calculate the system size in kW using: kW = annual kWh / (peak sun hours/day x 365 x system efficiency). Then divide by the wattage of a single panel (typically 350 to 420 W). For example, a 7 kW system using 400 W panels needs 17 to 18 panels.
What is a peak sun hour?
A peak sun hour is one hour of sunlight at an intensity of 1,000 watts per square meter (1 kW/m2). The number of peak sun hours per day varies by location: Phoenix, AZ averages about 6.5; Denver, CO about 5.5; New York about 4.7; Seattle about 3.8. NREL's PVWatts tool provides precise data for any US location.
What does system efficiency include?
System efficiency (also called performance ratio or derate factor) accounts for inverter losses, wiring resistance, soiling, shading, temperature effects, and module mismatch. A typical residential system operates at 75 to 85% of rated DC capacity. The DOE commonly uses 0.77 as the default derate factor.
Should I size my solar system to cover 100% of my electricity use?
Most installers aim for 90 to 100% offset, but the optimal size depends on your utility's net metering policy, time-of-use rates, and whether you have battery storage. In states with 1:1 net metering, 100% offset is usually financially optimal. In states with reduced export rates, a smaller system paying off faster may be better.
What is the difference between system kW (DC) and kW-AC output?
System kW (or kWp) is the total rated DC wattage of all panels. The inverter converts this to AC with some losses (typically 95 to 98% efficiency). The AC output rating is typically 80 to 85% of the DC rating. Utilities and incentive programs may reference either; verify which rating applies to your interconnection agreement.
Official sources
- NREL PVWatts: PV Watts Calculator (National Renewable Energy Laboratory).
- U.S. DOE Solar Energy Technologies Office: Homeowner's Guide to Going Solar.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.