Charging an EV on Solar: How to Size Your System for Home Charging

Buying an electric car and putting solar on the roof are two of the best moves a homeowner can make — but only if the system is sized for both. Most solar sizing starts with last year's electricity bills. Add an EV and that baseline is already wrong: you're about to plug a new, large, daily load into the same house. The fix is a three-line math problem, and the U.S. Department of Energy's home-solar planning guide is explicit about the rule: size a system from your actual kWh usage reviewed month by month, and add any new load — including EV charging — to that baseline before sizing [2]. Here is how to do it.

Step 1: Find your household baseline in kWh

Every electric bill reports usage in kilowatt-hours (kWh) — the same unit installers use to estimate a system's energy production. The Energy Department guide advises reviewing bills month by month, since usage is seasonal and a single month says almost nothing [2]. Gather 12 months, add them up, and you have your baseline: the annual kWh your home uses today, before the car.

If you haven't bought the EV yet, this step matters most: size only to today's usage and you'll effectively be sizing for a house without a car — then paying for the car's electricity from the grid for the system's lifetime.

Step 2: Estimate the EV's annual electricity load

An EV's charging load is two of your own numbers multiplied together: how far you drive each year, and how much electricity your car uses per mile. Use real figures, not guesses — the DOE's Alternative Fuels Data Center is the official source for vehicle and charging-infrastructure data, so confirm your car's efficiency there rather than trusting dealer or marketing claims [1].

The formula is simple:
Annual charging kWh = annual miles driven × your car's electricity use per mile (kWh/mile)

Find the efficiency figure on your car's EPA fuel-economy label or the manufacturer's published specifications — it is typically stated per 100 miles, so divide by 100. Your annual miles come from your odometer history or the mileage estimate you gave your insurer. Both numbers are yours; the DOE's data center exists so you can verify the vehicle side from an official source [1].

Two caveats. First, charging isn't perfectly efficient: the electricity leaving your house meter exceeds what the car stores, because of wall-to-battery losses. Second, real-world efficiency is seasonal — cold weather and highway driving move it up. Treat your calculation as your meter-side load estimate, and share your assumptions with the installer so they can sanity-check them.

Step 3: Add the EV load to your baseline

Now you have a new target: baseline annual kWh + EV annual charging kWh. This is the number bids should be measured against. The Energy Department guide says bids should state the system's maximum capacity in watts (W) or kilowatts (kW) and its estimated annual energy production in kWh — and that the estimated-kWh figure is the one for comparing against your usage and your bills [2].

Because you've already converted your EV into the same unit, the comparison is direct: a bid whose estimated annual production roughly equals your adjusted total is sized to offset both house and car. For quote-to-quote comparison, the guide's two comparison metrics are $/watt and estimated $/kWh — not the total sticker price [2]. Comparing on $/watt keeps a bigger, EV-inclusive system honest against a smaller, house-only bid.

When you charge can matter as much as how much

There is a timing problem hiding in this math. Solar panels produce during the day; most commuters charge at night. When exported daytime energy earns full credits against nighttime charging, the mismatch barely matters — but some newer rate designs credit daytime exports at a fraction of the nighttime rate you pay to charge the car. So before you finalize the system size, pull your utility's published rate schedule and check how daytime exports and overnight charging are each priced.

On a time-of-use rate plan, charging the car after the peak window ends is cheap — but that also makes each kWh your panels offset elsewhere more valuable. That doesn't change the sizing method; it just means the dollar value of the offset deserves its own line in your payback thinking.

Check your electrical panel before you sign

An EV charger is often the largest new circuit a home ever gets. The Energy Department guide says to assess electrical panel capacity before committing — a site survey should confirm the panel has room for both the solar backfeed breaker and the charger circuit [2]. If the panel is full or undersized, an upgrade becomes part of the real installed cost, and it should appear in the bid — not discovered on install day.

Get this assessed during the solar site survey rather than after. Legitimate bids itemize everything, and the guide recommends comparing installers on itemized work and materials so nothing is bundled into an opaque total [2].

Mistakes to avoid

The most common one is sizing to last year's bills and "figuring out" the EV later — even though the Energy Department's sizing approach requires planned changes to be known up front [2]. The second is using the car's advertised range efficiency without checking the EPA label — manufacturer headline figures and real meter-side consumption are not the same thing, which is exactly why the DOE maintains official data to cross-check against [1]. The third is comparing bids on sticker price instead of $/watt and estimated $/kWh, the guide's recommended comparison metrics [2] — a system that covers house plus car will always cost more in total than a house-only system, and that's the wrong number to fear.

The bottom line

An EV doesn't make solar sizing complicated; it just makes one step mandatory that buyers without cars skip. Twelve months of bills, your car's real efficiency from official data, one multiplication — and you walk into the installer visit with a target every bid can be judged against.

Sources

• [1] U.S. Department of Energy — Alternative Fuels Data Center (official source for alternative-fuel vehicle and charging-infrastructure data)
• [2] U.S. Department of Energy — Planning a Home Solar Electric System

Verified: 2026-10-12

Disclosure: This article was drafted with AI assistance and reviewed by a human editor. Figures reflect sources available on the verified date; confirm current program rules before acting.