Calculating the Number of Solar Panels Needed for a Home

Most homes we build for end up with fifteen to thirty panels. Yours depends on four numbers, and three of them are on your power bill.

Start with a year of bills, not a month

Add up the kilowatt-hours on twelve consecutive bills. Not the dollars, the kilowatt-hours, because rates change and usage is what the array has to cover. A July bill in the Salt Lake valley with the air conditioner running can be double a March bill, so a single month tells you nothing. In round numbers a Wasatch Front home often lands between 10,000 and 15,000 kilowatt-hours a year; Idaho homes on gas heat often run lower. Yours is whatever the bills say.

What a kilowatt of panels makes here

A well-placed kilowatt of panels in northern Utah or southern Idaho produces somewhere between 1,400 and 1,750 kilowatt-hours a year. South-facing, unshaded, at a typical roof pitch, sits near the top of that range. East or west facing, or a roof with afternoon shade, sits lower. This is the number the design software refines from your actual roof; for a first pass, use the middle.

The worked example

Say the bills add up to 12,000 kilowatt-hours a year. Divide by 1,600 kilowatt-hours per kilowatt. That is 7.5 kilowatts of panels. Today's residential panels are rated 410 to 440 watts each, so 7,500 watts divided by 425 watts is a little under 18 panels. Call it 18. A shaded roof might push that to 20. A perfect south roof might do it in 16.

Why we usually size smaller than that

Here is the part most online calculators skip. Under Rocky Mountain Power's net billing, power you export earns about 4.9 cents a kilowatt-hour in summer and about 4.0 cents in winter as of March 2026, and you buy power back at well above that. An array sized to cover every kilowatt-hour you use over the year will export heavily at midday in summer and sell that power cheap. So the design that pays best covers what the house uses while the sun is up and lets the grid handle the evening, unless you add a battery to shift the surplus into the evening yourself. That often means fewer panels than the worked example, not more. Idaho Power's program has its own rates and the same logic.

What changes the count

Adding an electric vehicle, a hot tub, or a heat pump raises the number. Replacing an old air conditioner or sealing the attic lowers it. If any of those is in your next three years, tell us before we size, because the array is easier to build right once than to extend later.

What to do with this

Run the example with your own twelve months and you will land within a few panels of what we would propose. The rest is the roof: direction, pitch, shade and room. Send us the bills and a photo and we will do the exact version.

Written and reviewed by the Intermountain Wind & Solar team. Reviewed September 2026.