How Many Solar Panels Does It Take to Charge an EV at Home

This page does the arithmetic from two numbers you can check elsewhere on this site: how much electricity an EV uses, and what a solar panel produces in a month. Nothing here comes from a sales sheet.

Start with the electricity the car uses

Our Level 2 EV charging guide gives the figure to start from: roughly 0.3 to 0.4 kWh for every mile driven. Multiply that by your monthly mileage. A 1,000 mile month therefore lands at 300 to 400 kWh. That monthly figure is the whole target, and everything after it is a conversion.

Work out what one panel produces in a month

Solar production is measured in peak sun hours, not daylight hours. A site with 4 peak sun hours receives the energy equivalent of 4 hours of full midday sun on a clear day. Four hours is a common planning value for the middle of the United States; the desert Southwest runs higher and the far North runs lower.

The arithmetic for one 400W panel at 4 peak sun hours, with a 0.8 derate for inverter losses, wiring losses and heat:

• 400W × 4 hours = 1,600 Wh per day at the panel
• 1,600 Wh × 0.8 derate = 1,280 Wh per day at the meter
• 1,280 Wh × 30 days = about 38 kWh per month

Divide the 300 to 400 kWh the car needs by 38 kWh per panel and you get 8 to 11 panels. At 400W each, that is 3.2 to 4.4 kW of extra array.

Change one input and the answer moves

The panel count is sensitive to peak sun hours, so run the same arithmetic at 3.5 hours and at 5.5 hours:

• 3.5 peak sun hours: about 34 kWh per panel per month, so 9 to 12 panels
• 4 peak sun hours: about 38 kWh per panel per month, so 8 to 11 panels
• 5.5 peak sun hours: about 53 kWh per panel per month, so 6 to 8 panels

If your commute is half the national average, halve the target. If you drive 2,000 miles a month, double it. The formula does not change.

What it costs, using this site's own $/W figure

The solar savings calculator uses about $3.00 per watt for installation. A 3.2 to 4.4 kW addition at that rate is $9,600 to $13,200 before any incentive. Check the federal tax credit guide for the current rules before you use that number in a budget, because the residential credit has changed and older guides still describe the old version.

If you already have solar, the marginal cost is usually lower than the same capacity bought as a new system, because the inverter, the permits and the crew are already there. Ask the installer who did the first job before you ask anyone else.

Do not oversize the array for the car

Net metering caps decide how much of your production the utility will credit. Our panel buying guide describes one installer proposing a system oversized by about 30 percent that would have produced power the owner could not get credit for. Size the array to the annual kWh you will actually use, EV included, and check the cap with your utility first.

Charge in daylight if you can

A panel produces nothing at night, so the car and the array only meet when the car is plugged in during the day. Without a battery, the evening charge comes from the grid. Our charging guide shows a time-of-use plan at 8 cents per kWh off-peak against 22 cents on-peak, which cut one bill from $52 a month to about $27. If your utility pays little for exported solar, overnight charging on a cheap rate can beat exporting daytime production.

The short recipe

1. Write down the kWh the car uses in a month.
2. Look up the peak sun hours for your location.
3. Multiply panel watts by peak sun hours by 0.8 by 30 to get monthly kWh per panel.
4. Divide the car's kWh by that number and round up.
5. Check the net metering cap before you order the extra panels.