This is the number every panel count, battery size and payback estimate is built on top of, and it is the one most homeowners never actually look up. Installers frequently estimate it from square footage and regional averages — which is fine for a first conversation and wrong often enough that it is worth ten minutes of your own time to check.
The only genuinely accurate method: twelve months of bills
- Log into your utility's online account. Nearly all of them keep 12-24 months of billing history under "usage" or "billing history."
- Record the kWh consumed for each of the last 12 months — not the dollar amount, which moves with rate changes and does not convert cleanly.
- Add the twelve figures together for your annual kWh.
- Divide by 365 for your average daily kWh.
Twelve months, not one. A single bill is close to useless for this: a home with central air and electric heat can easily use three times as much electricity in August as in April. Sizing from a spring bill undersizes the array badly; sizing from an August bill oversizes it just as badly.
Worked example
| Month | kWh | Month | kWh |
|---|---|---|---|
| January | 1,150 | July | 1,320 |
| February | 1,020 | August | 1,380 |
| March | 820 | September | 1,090 |
| April | 660 | October | 710 |
| May | 690 | November | 790 |
| June | 1,010 | December | 1,060 |
Annual total: 11,700 kWh. Daily average: 11,700 ÷ 365 = 32 kWh per day. Note the shape of that curve — the peak month is more than double the lowest month, which is exactly why the annual figure is the one to size against.
Benchmarks, if you cannot get twelve months of bills
| Household | Monthly kWh | Daily kWh |
|---|---|---|
| 1-2 people, apartment or small home, gas heat | 400 – 600 | 13 – 20 |
| 3-4 people, average home | 850 – 1,000 | 28 – 33 |
| Large home, electric heat or an EV | 1,400 – 2,500 | 46 – 82 |
The U.S. residential average is roughly 10,500-10,800 kWh a year, or about 900 kWh a month. Use these only as a fallback — the spread within any single row is wide enough that a real bill beats a benchmark every time.
Adjust for what is about to change
Historical usage answers what your house has consumed. Size for what it will consume over the array's 25-year life:
- An electric vehicle adds roughly 3,000-4,000 kWh a year for typical mileage — often the single largest change a household makes
- A heat pump replacing a gas furnace adds roughly 2,000-4,000 kWh a year depending on climate
- A pool pump adds roughly 1,500-3,000 kWh a year
- Efficiency upgrades (insulation, LED conversion, a newer HVAC system) subtract — do these before sizing solar, since reducing consumption is almost always cheaper per kWh than generating it
- Household changes — a child moving out, or a partner starting to work from home, both move the number more than people expect
Size for committed changes, not speculative ones. With the federal 30% credit gone, buying panels for an EV you might get in four years is now considerably more expensive than leaving roof space and adding a string later.
Daily kWh and peak watts are two different problems
Your daily kWh figure sizes the panel array — how much energy you need to generate. It says nothing about how much power you need at once, which is what sizes an inverter or battery output. A house using 32 kWh a day might peak at 8,000W when the AC compressor and the oven run together, and an inverter sized to the daily average would trip instantly.
Use this number for the array, then handle the instantaneous side separately with our free wattage calculator, which adds up running and surge watts appliance by appliance. Once you have your annual kWh, how many solar panels do I need turns it into a panel count.