Solar Calculator: Estimate System Size, Cost and Payback

This calculator gives you a rough idea of how big a solar system your home would need, what it might cost and how long it could take to pay for itself. It runs entirely in your browser, and nothing you type is saved or sent anywhere.

Your home

Use your average over a full year if you can. Your bill includes fixed fees, so this slightly overestimates your usage.
Filled in with your state’s average residential rate for January to July 2026 (EIA). Your bill shows your real rate.
Rough adjustments. A site visit or a tool like PVWatts gives a better number for your exact roof.
Percent
US average $2.60 before incentives (EnergySage, Sept 2026)
The 30% federal tax credit ended for home systems installed after 2025, so it isn’t included.
More assumptions
Most states still offer some form of net metering. California moved new customers to net billing in 2023, so it’s preselected there.

Your estimate

–System size
–Panels
–Cost after rebates
–Payback
  • Your yearly use–
  • Expected production, year one–
  • Cost before rebates–
  • Savings, year one–
  • Net savings over 25 years–
  • Roof space needed (about)–
Savings minus what you paid, by year

Cash purchase, before any loan interest, inverter replacement or maintenance. Ballpark only: real quotes depend on your roof, equipment, installer and utility rules.

How the estimate works

Your usage. If you enter a monthly bill, the calculator divides it by your electricity rate to estimate how many kilowatt-hours (kWh) you use. The default rate is your state's average residential price for January through July 2026, from the U.S. Energy Information Administration. Your own bill will show your actual rate, and it's worth using that instead.

Sunlight. Each state uses long-term average sunlight (2001 to 2020) for one city, taken from NASA's POWER project, for a south-facing surface tilted about 15 degrees less than the latitude. The calculator then takes off 23% for real-world losses such as heat, wiring, dirt and the inverter. Sunlight can differ a lot within a big state, so treat this as a starting point.

System size and cost. Size is your yearly use, times the share you want to cover, divided by what one kilowatt of panels should produce in a year. It's rounded up to whole panels. Cost is the size times the price per watt. The default of $2.60 per watt is EnergySage's national average for September 2026. State averages in the same data ran from about $2.12 in Florida to $3.81 in Alabama.

Savings. Each kWh the panels produce, up to your yearly use, is counted at your electricity rate. If you pick net billing, only the share you use as it's produced gets the full rate, and the rest earns the export credit you enter. Panel output is assumed to drop 0.5% a year, which is the median rate NREL researchers found across many systems. Electricity prices rise 3% a year by default. For reference, the U.S. average residential price rose about 3.6% a year from 2016 to 2025.

Payback and 25-year savings. Payback is the point where your total savings equal what you paid. The 25-year figure is total savings minus the system cost, in today's dollars with no discounting.

What it leaves out

  • Loan interest and fees. A 2024 Consumer Financial Protection Bureau report found that hidden "dealer fees" on solar loans often add 10% to 30% to the price.
  • Leases and power purchase agreements, where you don't buy the panels. Your savings then depend on the contract rate and its yearly increase. See can you get free solar panels for how those deals work.
  • Batteries. A typical 13.5 kWh home battery costs around $15,650 installed. Solar backup batteries covers when one makes sense.
  • Inverter replacement and upkeep. A string inverter often needs replacing once during a system's life, usually for $1,000 to $3,000. More in what solar maintenance costs.
  • Fixed monthly charges, minimum bills, time-of-use rates and future changes to your utility's solar rules.

Getting a better number

For a closer estimate for your exact roof, try PVWatts, a free tool from the National Laboratory of the Rockies (formerly NREL). You can enter your address, roof angle and direction.

After that, the most useful step is getting a few quotes and comparing them on price per watt, equipment and warranties. The first steps to going solar walks through it, and how long solar takes to pay for itself explains what moves the payback period.

Sources