System sizing

Oversizing for a future EV

Buying an EV in the next year or two? Enter its expected mileage and efficiency to see exactly how many extra panels cover its charging, on top of your home usage.

Your numbers

1 Project details Optional
Shown on your exported report. Leave blank to skip.
2 Your current home usage
Add up 12 months of bills before the EV arrives, or your most recent full year.
3 Your planned EV
The US average driver covers about 12,000 to 14,000 miles a year.
Check the EPA window sticker or fueleconomy.gov. Those ratings are measured at the wall, so they already include charging losses. Most sedans run 3 to 4, trucks and larger SUVs run 1.5 to 2.5.
4 Your location and equipment
Check NREL's PVWatts for your address.
Most home panels quoted in late 2025 were 400 to 460 watts, according to EnergySage's marketplace data.
5 Derate factor

How it works

Three steps to your EV allowance

Step 1

Estimate the EV's annual electricity draw

Divide expected annual mileage by the vehicle's efficiency in miles per kWh. The EPA window sticker or fueleconomy.gov gives you this number even before you own the car, and it already includes charging losses.

Step 2

Add that load on top of your current usage

Combine your home's existing annual usage with the EV's estimated draw to get the true target your system needs to cover. Check the 110% offset calculator if you want a further buffer beyond just the EV.

Step 3

Tell your installer before the design is final

Building EV capacity into the original design almost always costs less than adding it later, since a second installation means a new mobilization and often a worse per-watt price.

Sizing solar around a car you don't own yet

Why "size to historical usage" leaves a real gap

An installer working from last year's utility bills alone has no way to know an EV is coming. If that system gets sized at exactly 100% of pre-EV usage, the household is left meaningfully undersized on day one, buying grid power for the car's charging that solar could have covered from the start. The EV charging load addition calculator breaks that new load down by specific vehicle and charging pattern, and the annual usage sizing tool shows the pre-EV baseline it builds on.

How much an EV actually adds

A typical EV driven 12,000 miles a year uses around 3,000 to 5,000 kWh annually, depending on efficiency and driving conditions. At a common 30 kWh per 100 miles, that works out to about 3,600 kWh a year, roughly a third of the 10,800 kWh the average US home uses in a year, according to EIA. Unlike a home battery, which mostly adds round-trip losses, as the future battery calculator shows, an EV adds real new load, so every mile you drive needs electricity your panels did not previously have to produce. At 4.5 peak sun hours and a 0.80 derate, that 3,600 kWh takes about 7 panels at 400 watts.

Why upfront capacity beats a second installation

Adding panels to an already-completed system means a new site visit, new permitting, and often a higher price per watt than the same capacity would have cost as part of the original design. Larger arrays also tend to have a lower cost per watt overall, so building in the EV allowance from the start usually costs less than expanding later. When that EV charges matters too, since overnight charging on a time-of-use plan is priced differently from daytime solar, a difference the time of use rate savings calculator works out for your utility's rate schedule. If you also want a general growth buffer beyond one vehicle, the 110% offset calculator sizes it.

Trucks and larger EVs need a bigger allowance

Efficiency varies enormously by vehicle. The EPA rates the 2024 Ford F-150 Lightning Extended Range at 48 kWh per 100 miles on fueleconomy.gov, roughly double the 25 or so an efficient sedan like the Tesla Model 3 uses for the same distance. At 12,000 to 13,500 miles a year, that pickup uses about 5,800 to 6,500 kWh, against 3,000 to 3,400 kWh for the sedan. If a truck or large SUV is the specific vehicle being planned for, use its actual efficiency figure rather than a generic EV estimate.

Questions

Frequently asked

It depends on mileage, driving efficiency, and local sun hours. A typical EV driven 12,000 miles a year at 30 kWh per 100 miles uses about 3,600 kWh annually, which takes about 7 panels at 400 watts, or roughly 2.8 kW, at 4.5 peak sun hours and a 0.80 derate. The calculator may show one fewer extra panel because it rounds the combined total.

Divide your expected annual mileage by the EV's efficiency in miles per kWh, or multiply mileage by its kWh-per-100-miles figure. A car driven 12,000 miles a year at 3.3 miles per kWh uses about 3,636 kWh annually, a number your dealer or the EPA's window sticker can help you estimate before you even own the car.

Sizing for it now is usually cheaper. Adding panels to an already-installed system means a second mobilization, added permitting, and often higher per-watt costs than including the extra capacity in the original design. The main risk is a change of plans: if the car never arrives, the extra panels still produce, and exported energy may earn credits depending on your utility's rules. Telling your installer about a planned EV before the design is finalized costs nothing.

Yes, meaningfully. At 12,000 to 13,500 miles a year, an electric pickup rated near 48 kWh per 100 miles uses roughly 5,800 to 6,500 kWh annually, well above the 3,000 to 3,400 kWh a 25 kWh per 100 miles sedan uses over the same distance.

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