Solar Panel Calculator
Size a residential solar panel system by entering your monthly electricity usage, peak sun hours for your region (based on NREL data), panel wattage, and system efficiency. The calculator determines required system size in kW, panel count, roof space needed (approximately 21 sq ft per standard panel), estimated annual production in kWh, annual savings at your electricity rate, estimated system cost at $2.75/watt installed, and payback period after the 30% federal Investment Tax Credit (ITC). System efficiency (typically 75–85%) accounts for inverter losses, soiling, temperature derating, and shading. The federal 30% solar tax credit applies to residential systems installed through 2032 — it's a dollar-for-dollar reduction in tax owed, not a deduction.
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Top PickSolar Panels (400W) — Amazonfrom $149/panel* Affiliate links — we may earn a commission at no cost to you.
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How to Use This Calculator
- 1Enter your measurements in the fields above. Use the units shown next to each field (ft, in, etc.).
- 2Click Calculate to get your material estimate instantly.
- 3Review the highlighted result — it shows the key quantity you need to order.
- 4Use Copy Results to save your estimate, or bookmark the URL — your inputs are encoded in the link.
Pro tip: Always add 10% to your material order to account for waste, cuts, and defects.
Frequently Asked Questions
How many solar panels do I need for a 1,500 sq ft house?
A 1,500 sq ft home uses roughly 600–900 kWh/month. At 5 peak sun hours and 80% system efficiency, a 100% offset system requires 5–7 kW = approximately 13–18 standard 400W panels.
What is the 30% federal solar tax credit?
The Residential Clean Energy Credit lets you deduct 30% of your total solar system cost from your federal income taxes. On a $20,000 system, that's a $6,000 tax credit. The credit applies to panels, inverters, labor, and battery storage installed through 2032.
How much roof space do I need for solar panels?
A standard 400W solar panel is approximately 66" × 44" (about 20–21 sq ft). A 6 kW system (15 panels) requires roughly 315 sq ft of unshaded south-facing roof area. Factor in roof obstructions, setbacks from edges, and shading from chimneys and dormers. Most installers require at least 250–400 sq ft of clear roof for a meaningful residential system.
What is a good payback period for solar panels?
The average US residential solar payback period is 6–12 years after the 30% federal tax credit. States with high electricity rates (California, Massachusetts, New York) see paybacks of 4–8 years. States with low electricity rates and fewer incentives may see 10–14 year paybacks. Solar panels typically carry 25–30 year performance warranties, so even a 12-year payback yields 13–18 years of free electricity.