☀️ SolarPanelEstimator.com

Solar Panel Calculator

Estimate panels, system size, and savings — no signup required

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How We Calculate Your Estimate

This calculator uses the PVWatts methodology developed by the National Renewable Energy Laboratory (NREL) and electricity rate data from the U.S. Energy Information Administration (EIA).

Step 1: Convert monthly bill to kWh (if needed)

monthlyKwh = monthlyBill ÷ utilityRate

Step 2: Annual consumption

annualKwh = monthlyKwh × 12

Step 3: System size

systemSizeKw = annualKwh ÷ (sunHours × 365 × orientationFactor × shadeFactor × 0.80)

Step 4: Actual production (after rounding up panel count)

annualProduction = panelCount × wattage × sunHours × 365 × orientFactor × shadeFactor × 0.80

Step 5: Financials

annualSavings = annualProduction × utilityRate

paybackYears = (systemKw × 1000 × costPerWatt) ÷ annualSavings

Step 6: CO₂

co2OffsetLbs = annualProduction × 0.827

The 0.80 derate factor accounts for real-world losses including inverter efficiency, wiring resistance, temperature effects, and soiling. This aligns with PVWatts default assumptions.

The 0.827 lbs CO₂/kWh factor is the EPA's national average grid emissions intensity for residential electricity.

Sun hours are NREL annual peak sun hour averages per state. Electricity rates are EIA 2024 residential averages. Cost per watt is sourced from Lawrence Berkeley National Lab's Tracking the Sun dataset.

Frequently Asked Questions

How accurate is this solar panel calculator?▾
This calculator uses NREL peak sun hour data and EIA electricity rate averages to provide a ballpark estimate. Real-world results vary based on your exact roof pitch, local shading, panel brand, and installer pricing. Treat this as a planning tool — get 2–3 installer quotes for precise numbers.
How many solar panels does the average home need?▾
Most US homes need between 15 and 30 panels (6–12 kW system). The exact number depends on your electricity usage, your state's sun hours, roof orientation, and the wattage of the panels you choose. Enter your bill or usage above to get a personalized estimate.
What is a good payback period for solar panels?▾
The typical solar payback period in the US ranges from 6 to 12 years. States with high electricity rates (California, Massachusetts, Hawaii) tend to have shorter payback periods. The 30% federal Investment Tax Credit (ITC) can shave 2–3 years off the estimates shown here.
Does roof orientation affect how much power solar panels produce?▾
Yes, significantly. South-facing roofs receive the most direct sunlight and are ideal (factor: 1.00). East and west-facing roofs produce about 15% less. North-facing roofs produce roughly 30% less. Flat roofs can be angled for optimal output.
What is the federal solar tax credit (ITC) in 2025?▾
The Inflation Reduction Act extended the Investment Tax Credit at 30% through at least 2032. This means you can deduct 30% of your total solar installation cost from your federal taxes. Our payback estimates do not include the ITC — factor it in to improve your actual payback period.

Related Resources

Disclaimer: This is an estimate for planning purposes only. Actual solar system size, cost, production, and savings depend on site-specific conditions including roof pitch, local shading, utility rate structures, equipment choices, installation costs, and available incentives. This tool does not constitute financial, tax, or engineering advice. Always consult a licensed solar installer and tax professional before making purchasing decisions. Data sources: NREL PVWatts, EIA Residential Energy Survey, EPA eGRID.