Start with your bill
Before you get a single solar quote, grab your electric bill. It holds the most important number for sizing a system: how many kWh your home uses in a year. Add your EV's needs on top, and you'll know roughly how big a system you need, and whether a quote makes sense.
Here's how to do it in four steps, with an example.
Step 1: Find kWh, not dollars
Electric bills are busy: delivery charges, fuel charges, fixed fees, taxes. For solar, ignore the "Total Amount Due." Solar panels make electricity, not money. Look instead for the number marked kWh (kilowatt-hours). That's how much energy you used during the billing period.
Step 2: Get a full 12 months
One month can be badly misleading. Air conditioning can run all summer, and heating may run all winter, while spring and fall are usually much lower.
Look for the bar chart on your bill, often on page 2, labeled something like "Energy Usage History" or "13-Month Usage." Add up the last 12 months. Many utilities also let you download your data from their website. Look for a Green Button "Download My Data" option, a standard backed by the U.S. Department of Energy.
Already have solar? Your bill may only show the power you bought from the grid, not what your home actually used. Add your solar app's production to get your real total.
Step 3: Add your EV
Multiply the miles you drive a year by your car's energy use. You can find the rating in kWh per 100 miles on fueleconomy.gov; divide it by 100 to get kWh per mile. For example, an efficient electric SUV like the Tesla Model Y Long Range uses about 0.29 kWh per mile. At 12,000 miles a year, that's about 3,480 kWh.
Your home's yearly kWh + your EV's yearly kWh = your target solar production.
Step 4: Turn kWh into a system size
Divide your target by how much 1 kW of panels makes in a year where you live. With average US sunshine (about 4.98 peak sun hours a day, the figure our calculator uses) and typical 18% system losses, 1 kW makes about 1,490 kWh a year. Sunnier states make more; Texas, for example, makes about 1,560.
So a 13,500 kWh target needs about 9 kW in an average-sun state. At $3.00 per watt with no tax credit, that's roughly $27,000 for home and car together. Our 50-state study lists the sunshine for every state.
An example bill, worked through
Here's an example household. Its August bill showed 1,500 kWh, but the usage chart told the full story. Fixed charges, taxes and delivery fees are left out, because only kWh matters for sizing. The EV is a 2024 Tesla Model Y Long Range (29 kWh per 100 miles, EPA).
| From the bill's usage history | kWh | Notes |
|---|---|---|
| Summer (June–August) | 4,500 | High use from air conditioning |
| Rest of the year (September–May) | 5,500 | Milder weather, lower use |
| Home's yearly use today | 10,000 | Your starting point, without an EV |
| Plus the EV | + 3,480 | 12,000 miles × 0.29 kWh per mile |
| Target yearly solar production | about 13,500 | Home + car |
The common mistake
Now compare that with the common mistake: taking the 1,500 kWh August bill and multiplying by 12. That gives 18,000 kWh a year for the home, about 8,000 kWh more than it really uses. With average sunshine, that's roughly 5.4 kW of extra panels, or about $16,000 you'd pay for and not need.
Mistakes to avoid
Multiplying one month by 12: summer and winter bills exaggerate; spring and fall understate. Always use a full year.
Forgetting future changes: planning to switch from a gas furnace to an electric heat pump, or buy a second EV? Your past bills won't show that. Add an estimate for it.
Ignoring time-of-use: your bill may split kWh into peak and off-peak. Total kWh decides how many panels you need, but timing affects whether a battery or a new charging schedule makes sense.
Overlooking size limits: some utilities and states cap system size, often at about 100% to 120% of your past 12 months of use. If you haven't bought the EV yet, ask how to include it; some utilities accept proof of purchase or a written load estimate.
An example: the salesperson and the July bill
Daniel, a homeowner in Texas, just bought a Ford F-150 Lightning and wanted solar to cover it. His July bill showed 2,000 kWh, because the air conditioning had been running all day.
A door-to-door salesperson took that bill, multiplied by 12, and assumed Daniel used 24,000 kWh a year. He offered a system of about 15 kW, roughly $45,000.
Daniel checked his 12-month usage history online instead. His home actually used about 11,000 kWh a year, because spring and fall were much lower. He added about 4,300 kWh for the truck: roughly 9,000 miles a year at 48 kWh per 100 miles. That's a target of about 15,300 kWh.
With Texas sunshine, 1 kW makes about 1,560 kWh a year, so he needed about 10 kW, roughly $30,000. Reading his own bill saved him about $15,000 of panels he didn't need. And because Texas has no statewide net metering, he also compared solar buyback plans before signing.
(Daniel is an example, not a real customer.)