How Many Solar Panels Do I Need? (2026)

You do not start with a panel count. You start with the electricity you want to cover and with how much a kilowatt of panels produces on that roof. Panel count is the last division: system watts divided by the wattage of the module on the quote. The solar calculator stops at system size and cost. This guide does the last step without pretending there is a national module size. That wattage is unverified here, so it stays an input.

From the bill to a system size

If you know monthly kWh, annual kWh is that number times 12. If you know only the dollar bill, monthly kWh is the bill divided by your rate per kWh. A rate of zero makes that division impossible, and the calculator says so. System size in kW is annual kWh divided by the production factor. The factor's unit is kWh per kW per year. Gross cost is system kW times 1,000 times cost per watt. Do not round the kW figure and then multiply if you want the same cents the calculator shows. It multiplies first and rounds the money after.

Work the example the calculator uses, and keep the label "example" on it. 1,200 kWh a month is 14,400 kWh a year. A factor of 1,400 produces 14,400 ÷ 1,400 = 10.2857... kW, which displays as 10.29 kW. At $3 per watt the gross price displays as $30,857.14. The factor 1,400 is not a US average published by this site. The EPA and NREL figure we do cite is different and is not a roof: 1,455,726 kWh from 1,017.14 kW in a Kansas City open-rack example, about 1,431 kWh per kW-year. Use it as a worked division of two sourced inputs. Use PVWatts for the house.

Then divide by the module on the quote

System watts are kilowatts times 1,000. The example's unrounded size is 14,400 ÷ 1,400 kW, which is 10,285.71 watts. If the quote's modules are 400 watts each, 10,285.71 ÷ 400 = 25.71 modules. You cannot buy 0.71 of a module. Buying 26 is a purchasing choice so the array is at least the modeled size. Buying 25 leaves you slightly short of the kWh target. Say which you did. The 400-watt number is this sentence's hypothetical quote, not a Calcledge benchmark.

If another quote uses 450-watt modules, the same 10,285.71 watts is 22.86 modules, so 23 panels. The roof did not shrink. The module got larger. Comparing "22 panels" with "26 panels" without the wattage is how two bids get misread. Ask both installers for kW DC, module watts, and the production factor, and put the kW and the factor into the calculator yourself.

The roof can reject the neat count

A 26-panel layout may not fit. Vents, hips, shade at 3 p.m., and a fire setback cut the usable area. If the fitter says the roof holds 20 of those 400-watt modules, the system is 8.00 kW, not 10.29. Annual production at a factor of 1,400 would be 8 × 1,400 = 11,200 kWh, not 14,400. You then cover less of the bill. Recalculate. Do not keep the 10.29 kW cost after the layout changed.

Shade belongs in the factor, not in a folk percentage. A tree that shades the south array in winter is a PVWatts shading loss or a different layout, not a number this site should invent. East and west splits produce less than a south array at the same tilt in many locations. Let the model say how much. The payback page explains what you do with the kWh once you trust them: payback explained.

Cost follows size, incentives follow the paperwork

More panels raise gross cost in a straight line only if cost per watt stays put. It often does not. A small system still pays for a permit and a truck roll, so the dollar-per-watt on a 4 kW job can be worse than on a 10 kW job. Type the quoted rate. Do not scale the example's $3 as if it were a law.

The federal residential credit is 30 percent only for qualified property installed from 2022 through December 31, 2025. The IRS says it is not available for property placed in service after that date. Panel count does not revive it. State incentives are unverified here, which is why the state menu is off. The guide on whether panels pay for themselves shows a labeled 0 percent case and a labeled 30 percent what-if so the two are not confused.

Bring the installer three questions: the module model and its nameplate watts, the kW DC they will actually fit, and the annual kWh from a model of this roof. Divide watts by nameplate watts yourself. If their panel count does not match, someone rounded early or dropped a module for a vent. Either way you want the reason before you sign. Financing, if any, comes after the size is real. Cash, loan, and lease are ways to pay for a system you have already sized, not ways to decide the panel count.