Solar Panel Calculator
Calculate how many solar panels you need, system size in kW, annual savings, payback period, and 25-year ROI. Includes tax credit, SREC value, and CO₂ offset.
$
$/kWh
hrs
W
System Size Needed
—
Number of Panels —
Estimated Annual Savings —
Extended More scenarios, charts & detailed breakdown ▾
$
System Size
—
Panels (400W) —
Annual Savings —
Professional Full parameters & maximum detail ▾
kW
kWh
$
%/yr
$
%
yrs
Production
Year 1 Production —
25-Year Total Production —
Financial
25-Year Total Savings —
25-Year ROI —
Monthly Loan Payment —
Environmental
CO₂ Offset per Year —
How to Use This Calculator
- Enter your monthly electricity bill and rate per kWh.
- Set your peak sun hours (find yours at pvwatts.nrel.gov) and panel wattage.
- Results show system size, panel count, and annual savings.
- Use Financial tab to enter system cost and see payback after the 30% federal tax credit.
- Use the Professional tab for 25-year production forecast, SREC income, CO₂ offset, and loan payment.
Formula
System size (kW) = (Monthly kWh ÷ 30 ÷ Sun hours) ÷ 0.80
Panels = System kW × 1000 ÷ Panel wattage
Net cost = System cost × (1 − 0.30 ITC)
Example
Example: $150/month bill at $0.13/kWh = 1,154 kWh/month. With 5 sun hours: 1,154÷30÷5÷0.8 = 9.6 kW system. At 400W/panel = 24 panels. Annual savings ≈ $1,620.
Frequently Asked Questions
- Divide your monthly kWh usage by the number of peak sun hours per day and by 30 days, then divide by panel efficiency (80%) and panel wattage. A typical US home uses 900 kWh/month and needs 20–25 panels.
- The Investment Tax Credit (ITC) offers 30% of the total system cost as a federal tax credit for systems installed through 2032. On a $20,000 system, that's $6,000 back.
- The average payback period in the US is 6–10 years, depending on electricity rates, sun hours, and incentives. After payback, electricity is essentially free for the remaining panel life.
- Yes, but at reduced output — typically 10–25% of full capacity on overcast days. The 80% efficiency factor in this calculator accounts for clouds, shading, heat losses, and inverter efficiency.
- Net metering allows you to sell excess solar electricity back to the grid, receiving a credit on your bill. Availability and rates vary by utility and state.