Solar Panel Calculator
Size a home solar photovoltaic (PV) array to offset your electric bills. Calculates total system kilowatts, required panel count, and roof space utilizing standard NREL solar performance factors.
Average monthly electricity usage from your utility bill (US average ~890 kWh)
Equivalent hours of full sun (1,000 W/m²) for your region (typically 3.5–5.5 hrs)
Individual solar panel power rating (modern residential panels are ~400W)
Total nameplate DC system capacity needed to meet 100% of demand
How to Use This Calculator
- Find your 12-month average monthly kilowatt-hour (kWh) usage on your electric bill.
- Enter your location's average daily peak sun hours.
- Select the wattage rating of your intended solar panels (typically 400W).
- Review required system kilowatts, module count, and roof area needed.
Formula & Calculation Methodology
Daily electric consumption is divided by peak sun hours and an industry-standard 0.77 derating factor (accounting for inverter conversion, wire loss, heat, and dirt) to size the DC array.
Worked Example
Average Home in Moderate Sun (4.5 Sun Hours)
Efficient Home in Sunny Desert (5.5 Sun Hours)
Frequently Asked Questions
What is a peak sun hour?↓
A peak sun hour is an hour during which solar irradiance averages 1,000 W/m², rather than simply daylight hours.
What does the 0.77 derate factor cover?↓
It accounts for real-world DC-to-AC inverter losses, panel temperature coefficient drops, wiring resistance, and dust soiling.
Which roof orientation is best for solar panels?↓
South-facing roofs maximize annual kWh production in the Northern hemisphere, while west-facing panels help offset peak summer afternoon utility rates.
Do solar panels produce power on overcast days?↓
Yes, solar panels produce roughly 10% to 25% of rated output under heavy clouds via diffuse ambient solar radiation.
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Source Citation & Disclaimer
Informational tool only; not licensed tax, legal, or financial advice.