Solar + EV: charging a car on sunshine

A solar-charged mile does not pay you the price of gasoline. It pays you your electricity rate, same as every other kilowatt-hour on the roof. That correction cost this calculator about $1,300 a year on its own flagship example, and it was the right call.

Open the calculator →Tick "Add an electric car (gas conversion)" in the Household section, then enter miles, MPG and MPGe.

Just want to know how many panels the car needs, what it saves or its carbon? Use the electric car calculator.

Why not gasoline

This tool used to credit solar with the gas a solar-charged EV avoided. At 28 MPG and $5.50 a gallon that makes a solar mile look like $0.50–0.70 per kilowatt-hour, nearly double a retail rate, and it flatters solar enormously. A reader took the argument apart and they were right.

You get the gasoline savings from owning an electric car. Not from owning panels. Sell the panels and keep the car, charge at midnight on grid power, and the gas savings are still there almost untouched. They were never solar's to claim.

What the car actually changes

None of it is the price of gas.

It makes the house bigger. A car adds roughly 3,000 kWh a year. That changes what size array makes sense, and in tiered-rate territory it can change which tier your last kilowatt-hour lands in.

It can eat noon. The calculator asks when the car charges, and its default is what a charger's "charge from solar" setting does: from the panels while the sun is up, the rest in the cheapest night hours, never from the grid at peak prices. Those kilowatt-hours get used on site instead of exported at a few cents. On net billing that is most of the car's value to the roof.

It competes with a battery. Worth knowing before you buy both: a car charging at midday takes exactly the surplus a battery wanted. For a Marin home using 8,000 kWh a year with a west-facing 4.6 kW system on PG&E's E-ELEC, a 13.5 kWh battery gives back about 2,400 kWh a year with no car, and about 1,200 with a car charging from the panels. The car is the cheaper way to absorb your own production, and it does it by driving somewhere.

Where the gas saving went

It did not disappear. Switching from gas to electric still saves real money, and the calculator still computes it: your gallons at your price, minus what the electricity costs. It sits on its own line on the vehicle card, escalating at a fuel-price outlook rather than your utility's rate, because gasoline moves differently from a regulated tariff.

Solar's own payback and equivalent return never include a drop of gasoline.

What to enter

Annual miles, your current car's real-world MPG (not the sticker), and the EV's MPGe. 120 is reasonable for an efficient sedan; a full-size electric truck is nearer 70, which is a bigger difference than most people expect when they are sizing an array around a vehicle they haven't bought yet.

Run your numbers →Or read How it works for the EV and capture assumptions in full.