How to Size Your Solar System From Your Electric Bill (10-Minute Math)

Every solar quote you will ever get starts with the same question: how much electricity does your house actually use? Not how big your roof is. Not what your neighbor installed. How many kilowatt-hours (kWh) you burn in a year.

Here's the good news: you already have that number. It is sitting in your last 12 electric bills. And with about ten minutes of arithmetic, you can size your own system before an installer ever walks through your door. That makes you a much harder customer to oversell.

This is the exact math, with real worked numbers you can run against your own bills tonight.

Step 1: Pull your annual kWh from the bills

Grab your last 12 monthly bills, or log into your utility's website, where most utilities show a 12 or 24 month usage history. You want the kWh consumed each month, not the dollar amount. Add all twelve together to get your annual consumption.

Twelve months matters. A single July bill will oversize your system if you run air conditioning; a March bill will undersize it. Usage in most homes swings 30 to 50 percent between the cheapest and most expensive months, so a one-month snapshot is not a sizing number.

If you only have dollar amounts, estimate kWh by dividing the energy part of the bill by your rate per kWh:

Monthly kWh = (bill amount - fixed charges) / rate per kWh

So a $170 bill with a $12 fixed connection charge, at $0.17 per kWh, works out to about (170 - 12) / 0.17 = 929 kWh. Check your actual rate on the bill itself. The national average is roughly 16 to 18 cents per kWh, but rates run from under 11 cents in parts of the South to over 30 cents in California and New England.

For context, the U.S. Energy Information Administration (EIA) puts the average household at about 875 kWh per month, or roughly 10,500 kWh per year, with an average bill near $155 per month. Your number may be higher or lower, but it is a reasonable benchmark to sanity check your own total against.

Step 2: The sizing formula

Once you know your annual kWh, the core formula is:

System size (kW) = daily kWh / (peak sun hours x 0.86)

Breaking that down:

• Daily kWh = annual kWh / 365
• Peak sun hours is a measure of your location's usable solar resource. It is the equivalent number of hours per day of full-strength (1,000 W/m2) sunlight. It is not hours of daylight. Most of the continental U.S. falls between 3.5 and 6.5.
• 0.86 is the derate factor. Real systems lose roughly 14 percent of their theoretical output to inverter conversion, wiring, panel soiling, temperature, and panel mismatch. This is the default loss assumption NREL uses in its PVWatts calculator, and it is a sound planning number.

Typical peak sun hour values by region:

• Desert Southwest (Phoenix, Las Vegas): 6.0 to 6.5
• Southern California, Texas: 5.0 to 5.75
• Florida, Southeast: 4.75 to 5.25
• Midwest, Mid-Atlantic: 4.0 to 4.7
• Northeast, Great Lakes: 3.8 to 4.3
• Pacific Northwest: 3.2 to 3.8

Step 3: A worked table from bill to system size

Let's run the whole chain, bill to kWh to system size, for a range of monthly bills. Assumptions here: $0.17 per kWh, 4.5 peak sun hours (a reasonable mid-country value), and the 0.86 derate.

• $100 bill = 588 kWh/month = 19.3 kWh/day = about 5.0 kW = 13 panels
• $150 bill = 882 kWh/month = 29.0 kWh/day = about 7.5 kW = 19 panels
• $200 bill = 1,176 kWh/month = 38.7 kWh/day = about 10.0 kW = 25 panels
• $250 bill = 1,471 kWh/month = 48.4 kWh/day = about 12.5 kW = 32 panels
• $300 bill = 1,765 kWh/month = 58.1 kWh/day = about 15.0 kW = 38 panels

(Panel counts assume roughly 400 W panels, so divide kW by 0.4.)

Let's follow one row through by hand so the table is not a black box. Take the $150 row. $150 / $0.17 = 882 kWh per month. Divide by 30.4 days = 29.0 kWh per day. Then 29.0 / (4.5 x 0.86) = 29.0 / 3.87 = 7.5 kW. At 400 watts per panel, that is about 19 panels.

Your numbers will differ with your rate and your sun hours. That is the point of doing it yourself. A $150 bill in Phoenix (6.5 sun hours, often cheaper power) needs a much smaller array than the same $150 bill in Seattle.

Step 4: Decide your offset target

The table above sizes for a 100 percent offset, producing as much annually as you consume. That is not automatically the right target for everyone:

• Under full net metering (your utility credits exports at the retail rate), 100 percent is usually the sweet spot. Going well beyond it rarely pays, because most utilities compensate annual surplus at a wholesale "avoided cost" rate of just a few cents per kWh.
• Under reduced export rates (like California's net billing rules), exported power is worth far less than power you use directly, which shifts the economics toward a somewhat smaller array, or toward adding a battery so you consume more of your own production.
• Planning an EV or a heat pump? Size for the load you will have in two years, not the load you have today. A typical EV adds roughly 2,500 to 3,500 kWh per year, which is about 1.5 to 2 kW of extra array in most of the country.

Step 5: Check the roof can hold it

Now, and only now, does the roof enter the picture. Each modern residential panel occupies roughly 20 square feet, so a 25 panel, 10 kW array wants about 500 or more square feet of usable, mostly south-facing or west-facing roof plane, keeping required setbacks from ridges and edges.

Shade is the other constraint. Panels in shade for chunks of the day produce a fraction of their rating, and no formula fixes that. You either avoid the shade, trim what is causing it, or use panel level electronics (microinverters or optimizers) to limit the damage.

If the roof genuinely cannot fit the size you calculated, you install what fits and accept a partial offset. A system covering 70 percent of your usage still eliminates 70 percent of the energy portion of your bill.

Step 6: Sanity check with PVWatts

Before you talk to installers, run your numbers through PVWatts, a free calculator from the National Renewable Energy Laboratory (NREL). You enter your address and a system size, and it uses decades of local weather data to estimate monthly and annual production, accounting for your actual climate, tilt, and orientation.

The workflow: take the system size from your Step 3 math, plug it into PVWatts, and compare its annual production estimate against your annual kWh from Step 1. If PVWatts says the system produces within about 10 percent of your consumption, your sizing is solid. If it is way off, your peak sun hours assumption was wrong for your location, so trust PVWatts and adjust the size up or down until production matches your target offset.

This is also your best defense during the quoting process. When an installer proposes a system, ask what annual production they are modeling and check it against PVWatts yourself. Honest installers use the same NREL data and will land close. A proposal promising production far above the PVWatts estimate for that size and location deserves hard questions.

The ten-minute version

1. Add up 12 months of kWh from your bills.
2. Divide by 365 for daily kWh.
3. Divide that by (your peak sun hours x 0.86) to get system size in kW.
4. Divide by 0.4 to get an approximate 400 W panel count.
5. Confirm the roof fits it. Adjust for shade and future loads.
6. Verify with PVWatts before signing anything.

Do this before the first sales call and you will be comparing quotes against your own numbers instead of taking anyone's word for what your house needs.

See your own numbers

The two variables that most change the answer, your actual peak sun hours and your usable roof area, are both site specific and impossible to eyeball. Instead of guessing, you can get them for your exact address. Scan your home at https://solrscan.com for a $19 satellite based report that shows your annual sunshine hours, the number of panels your roof can actually fit, the recommended system size and install cost, and your projected energy savings. No sales call, no drive-by estimate. Just your real roof and your real numbers, so you walk into the first conversation already knowing your house.