Every solar quote you’ll ever get starts with the same question: how much electricity does your house use? Not how big your roof is, not what your neighbor installed — how many kilowatt-hours you burn in a year. Your electric bills already contain that answer, and with about ten minutes of arithmetic you can size your own system before an installer ever sets foot on your property.
That matters because you’ll be a much harder customer to oversell. When we sized our own 40-panel system, having done this math first meant we could push back on proposals instead of nodding along. Here’s the full process, with real worked numbers.
Step 1: Pull Your Annual kWh From Your 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 months together to get your annual consumption.
Twelve months matters. A July bill alone will oversize your system if you run air conditioning; a March bill will undersize it. Usage in most homes swings 30–50% between the cheapest and most expensive months.
If you only have dollar amounts, estimate kWh by dividing the bill by your rate:
Monthly kWh ≈ (bill amount − fixed charges) ÷ rate per kWh
So a $170 bill with a $12 fixed connection charge, at $0.17/kWh, works out to (170 − 12) ÷ 0.17 ≈ 929 kWh. Check your actual rate on the bill itself — the national average is around 16–17 cents, but rates run from under 11 cents in parts of the South to over 30 cents in California and New England.
Step 2: The Sizing Formula
Once you know your annual kWh, the core formula is:
System size (kW) = daily kWh ÷ (peak sun hours × 0.86)
Breaking that down:
- Daily kWh = annual kWh ÷ 365
- Peak sun hours is a measure of your location’s usable solar resource — 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 US falls between 3.5 and 6.5.
- 0.86 is the derate factor — real systems lose roughly 14% of their theoretical output to inverter conversion, wiring, panel soiling, temperature, and mismatch. This is the default loss assumption NREL uses in its PVWatts calculator, and it’s a sound planning number.
Typical peak sun hour values by region:
| Region | Peak sun hours/day |
|---|---|
| Desert Southwest (Phoenix, Las Vegas) | 6.0–6.5 |
| Southern California, Texas | 5.0–5.75 |
| Florida, Southeast | 4.75–5.25 |
| Midwest, Mid-Atlantic | 4.0–4.7 |
| Northeast, Great Lakes | 3.8–4.3 |
| Pacific Northwest | 3.2–3.8 |
Step 3: The Worked Sizing Table
Let’s run the whole chain — bill to kWh to system size — for a range of monthly bills. Assumptions for this example table: $0.17/kWh, 4.5 peak sun hours (a reasonable mid-country value), and the 0.86 derate.
| Avg monthly bill | Monthly kWh | Daily kWh | System size needed | ~Panels (400 W) |
|---|---|---|---|---|
| $100 | 588 | 19.3 | 5.0 kW | 13 |
| $150 | 882 | 29.0 | 7.5 kW | 19 |
| $200 | 1,176 | 38.7 | 10.0 kW | 25 |
| $250 | 1,471 | 48.4 | 12.5 kW | 32 |
| $300 | 1,765 | 58.1 | 15.0 kW | 38 |
Follow one row through by hand so the table isn’t a black box. Take the $150 row: $150 ÷ $0.17 = 882 kWh/month. Divide by 30.4 days = 29.0 kWh/day. Then 29.0 ÷ (4.5 × 0.86) = 29.0 ÷ 3.87 = 7.5 kW. At 400 watts per panel, that’s about 19 panels.
Your numbers will differ with your rate and your sun hours — that’s 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% offset — producing as much annually as you consume. That isn’t automatically the right target:
- Under full net metering (your utility credits exports at the retail rate), 100% is usually the sweet spot. Going 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 current 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 heat pump? Size for the load you’ll have in two years, not the load you have today. A typical EV adds roughly 2,500–3,500 kWh/year; that’s 1.5–2 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+ 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’s causing it, or use panel-level electronics (microinverters or optimizers) to limit the damage. Our own 40-panel system ended up spread across multiple roof planes precisely because the ideal single plane didn’t have enough clear area; that’s common, and it works, but each plane’s orientation slightly changes its output.
If the roof genuinely can’t fit the size you calculated, you install what fits and accept a partial offset. A system covering 70% of your usage still eliminates 70% 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. You enter your address and a system size; 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% of your consumption, your sizing is solid. If it’s way off, your peak-sun-hours assumption was wrong for your location — 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’re 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
- Add up 12 months of kWh from your bills.
- Divide by 365 for daily kWh.
- Divide that by (your peak sun hours × 0.86) to get system size in kW.
- Divide by 0.4 to get an approximate 400 W panel count.
- Confirm the roof fits it; adjust for shade and future loads.
- Verify with PVWatts before signing anything.
Do this before the first sales call and you’ll be comparing quotes against your own numbers instead of taking anyone’s word for what your house needs.