A bill and a sun-hour guess are a sketch. A roof needs a better model before you sign.
The solar calculator is a bill-to-payback sketch. It is not a roof design. The federal piece changed, and the page’s incentive box starts at 0% because of that. Here is the arithmetic, and the point at which you should stop trusting it.
A $180 monthly bill at $0.16 per kilowatt-hour is 1,125 kWh a month, about 37.5 kWh a day. Divide daily use by peak sun hours and by 0.8, which stands in for real-world losses. At 4.5 sun hours, the middle option on the page, that is about a 10.4 kW system. At $2.80 a watt the hardware is about $29,200. If the array really offsets the whole bill, year-one savings are $180 × 12, or $2,160. Payback at a 0% incentive is about 13.5 years. Type 30% and the net cost is about $20,400, with payback near 9.5 years. Only type 30% if you actually have a credit of that size.
The IRS Residential Clean Energy Credit page is the authority on the federal credit. For homeowner-owned systems installed after December 31, 2025, that 30% credit is gone. A system placed in service earlier, a credit you are carrying forward, or a state or utility rebate can still be a percent you type. A lease or a power-purchase agreement, where you do not own the panels, is a different contract. The Department of Energy’s homeowner guide is the plain-language version of who gets the credit and how not to get rushed into a signature.
Year-one savings equal the current bill. That is only true if the array covers your use, the roof is not shaded at 3 p.m., and you do not buy an electric car or a heat pump next summer. The sun-hour menu is three broad bands, not your roof’s azimuth. Production then grows with whatever utility-rate increase you type. A high increase makes any solar deal look brilliant. Use a number you would defend, or run it at 0% and again at 3% and see whether the decision survives.
For a roof, use PVWatts, the national-lab calculator. It takes a location, tilt, and direction and estimates kilowatt-hours. The catalog entry at OpenEI describes what that model is. If PVWatts says the roof will make 8,000 kWh and your bill is 13,500 kWh, this page’s “the whole bill” assumption is wrong and the payback is longer. Electricity prices, if you cannot find the cents on your bill, are published by the Energy Information Administration.
Thirteen years is a long time to be right about a roof, an inverter, and a rate. Nine years is still a long time. Neither number includes a new roof you should have replaced first, a service-panel upgrade, or the loan interest if you finance the system. Get two written production estimates and compare them with PVWatts before you compare them with this sketch. If you are also replacing the air conditioner or adding a car charger, size those loads first. The BTU calculator and the EV fuel page are the neighboring guesses, not a substitute for the installer.
Last updated: October 29, 2026. Estimates only. Not tax, legal, financial, or professional advice. See the terms.