Solar Panel Calculator
Enter your monthly electric bill to size a solar system — panels, cost, savings, and payback in one shot.
Runs entirely in your browser — your numbers never leave this page.
How it works
Solar sizing is backwards from your bill: usage → system size → panels → cost:
kW = (monthly kWh ÷ 30) ÷ (sun hours × 0.8)
- Sun hours = peak solar hours for your location (not daylight hours). NREL maps give your exact number.
- 0.8 derate accounts for inverter losses, heat, dust, and wiring — real systems produce ~80% of nameplate.
- 400W panels are the 2026 standard; count = system watts ÷ 400, rounded up.
- Cost: ~$2.80/watt installed is the national average — a 7 kW system ≈ $19,600 gross, ~$13,700 after the 30% federal credit.
- Payback: net cost ÷ yearly savings. 6–9 years is typical; high-rate states (CA, HI, MA) pay back fastest.
- Roof check: south-facing, unshaded, <10 years old. Old roof? Replace it first — removing panels to re-roof is expensive.
Frequently asked questions
How many solar panels do I need?
Divide your daily kWh by (sun hours × 0.8) to get kW, then panels = kW × 1000 ÷ 400. A $150/month bill ≈ 18–20 panels.
How much does solar cost?
About $2.80/watt installed before incentives — a typical 7 kW home system ≈ $19,600 gross, ~$13,700 after the 30% federal tax credit.
What is the federal solar tax credit?
30% of system cost as a tax credit for installations through 2032 (verify current law). It's a credit, not a deduction — it cuts your tax bill dollar for dollar.
How long is the payback period?
Typically 6–9 years. High electricity rates and good sun shorten it; cheap power and cloudy skies lengthen it.
Do solar panels work on cloudy days?
Yes, at reduced output — ~25–40% of sunny-day production. Snow and heavy overcast cut it further; annual estimates already account for weather.
How much roof space do I need?
About 18 sq ft per panel. A 20-panel system needs ~360 sq ft of usable, unshaded roof.
What if my roof is old?
Replace it first if it has under ~10 years left. Removing and reinstalling panels for a re-roof costs thousands.
Do panels need maintenance?
Very little — rain usually cleans them. Check production monthly via the inverter app; a sudden drop means soiling or a fault.
What happens at night / in outages?
Grid-tied systems shut off in outages (safety rule) unless you have batteries. Batteries add ~$10k+ but give backup power.
Should I lease or buy?
Buy if you can — leases and PPAs give you smaller savings and complicate home sales. Loans are the middle path: you own the system and the credit.