How the solar panel calculator works
Size a solar system from your electricity bill in under a minute. No signup, no email, and nothing leaves your browser — every number on this page is calculated on your device.
Usage first, then sun, then panels. The calculator turns your monthly usage into daily kWh (monthly kWh ÷ 30), then divides by your peak sun hours and by a system derate of 0.8 for real-world losses — inverter, wiring, temperature, soiling and shading. That gives the system size in kW. Panel count is system kW × 1000 ÷ panel wattage, rounded up, because you cannot buy a part of a panel.
For example, a US home using 900 kWh a month with 4.5 peak sun hours needs 30 ÷ 4.5 ÷ 0.8 = 8.33 kW, which is 21 standard 400W panels — an 8.4 kW system covering about 378 sq ft of roof (about 18 sq ft per panel). Yearly production is installed kW × sun hours × 365 × 0.8: about 11,038 kWh a year for this example, covering about 102% of the home’s annual use.
How cost and payback are estimated
Cost is system size × cost per watt. At a typical $2.90 per watt installed, the 8.4 kW example costs about $24,360. Yearly savings are the electricity you no longer buy (up to your annual use) plus exported surplus credited at half your retail rate — $1,856.20 a year ($154.68 a month) at $0.17 per kWh. Simple payback is cost ÷ yearly savings: 13 years 1 month, shown as a range of 10.74 to 16.04 years to reflect cost and price uncertainty.
All figures use post-credit maths: the US 30% federal residential solar credit ended on 31 December 2025. Savings are estimates, never guarantees — always confirm with local installers before you buy.
Worked example: a $153 monthly bill, step by step
Follow the calculator’s own defaults. A $153 monthly bill at $0.17 per kWh means 900 kWh a month, or 30.0 kWh a day. Divide by 4.5 peak sun hours and by the 0.8 derate: 30 ÷ 4.5 ÷ 0.8 = 8.33 kW. That rounds up to 21 panels at 400 W, an 8.40 kW system on about 378 sq ft of roof. It produces about 11,038 kWh a year — about 102% of the home’s 10,800 kWh annual use. It costs $24,360.00 to install, saves $1,856.20 a year, pays back in 13 years 1 month, and is estimated to save $63,968.98 over 25 years. Change any input and the calculator redoes the whole chain.
| Flagship worked example | Figure |
|---|---|
| Monthly electricity bill | $153 |
| Electricity rate | $0.17 per kWh |
| Monthly electricity use | 900 kWh |
| Daily electricity use | 30.0 kWh |
| Peak sun hours | 4.5 per day |
| System size needed (before rounding) | 8.33 kW |
| Panels | 21 × 400 W |
| Installed system size | 8.40 kW |
| Roof area | 378 sq ft |
| Yearly production | 11,038 kWh |
| Installed cost | $24,360.00 |
| Yearly savings | $1,856.20 ($154.68 a month) |
| Payback | 13 years 1 month |
| 25-year savings | $63,968.98 |
What changes the answer
Three inputs move the result more than anything else: your sun, your rate, and your roof. Five peak sun hours in Arizona produce roughly a third more electricity from the same panels than three and a half in the Pacific Northwest, so a system sized for Phoenix is oversized for Seattle. A higher electricity rate shortens payback fast — at $0.30 per kWh the same 8.4 kW system saves about twice as much a year as it does at $0.17. Roof direction and shade matter too: a south-facing, unshaded roof beats an east-facing or partly shaded one by a clear margin, and the 0.8 derate already assumes some loss. If you are about to buy an electric car or a heat pump, size for the usage you will have, not the usage on your last bill.
Related calculators: how many solar panels, solar panel cost, solar battery, solar savings and solar payback.
Sources
- U.S. Department of Energy — solar energy basics, system sizing and homeowner guidance.
- EnergySage — US solar costs, panel output and payback benchmarks.
- Solar Energy Industries Association (SEIA) — US solar market data and policy, including the federal tax credit.
By Shehzad Hussain. Published November 9, 2026. Last updated November 11, 2026.