Solar + EV: charging a car on sunshine

If a solar-charged EV takes the place of a gas car, the right way to value that energy isn't your electricity rate — it's the price of the gasoline you no longer buy. That single change moves solar payback more than almost any other input.

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

Why gasoline, not electricity

A kWh of solar you use at home offsets a kWh you'd have bought from the utility — worth your retail rate, maybe $0.30–0.40. The same kWh sent to an EV that replaces a gas car offsets a gallon-fraction of gasoline. Work it through: an efficient EV goes about 3.5–4 miles on a kWh; at 28 MPG and $5.50/gallon, those miles would have cost roughly $0.50–0.70 in gas. So a solar-charged mile is worth close to double a solar kWh spent on the house.

This is why a solar quote that only credits "avoided electricity" understates the system for anyone driving electric. The calculator sizes the EV's yearly energy need as miles ÷ MPGe × 33.7 kWh (33.7 kWh is the EPA's energy content of one gallon of gasoline) and values the solar that charges it at (miles ÷ MPG) × gas price.

How the EV folds into the result

• Captured solar is credited to the car first, then the house — the highest-value use of each kWh, counted once.
• Charging during peak sun raises capture. There's a separate toggle for "I can charge the car during peak solar hours" — that pulls the EV load into your roof's production window so it draws straight from the panels instead of overnight from the grid.
• Gas savings escalate on their own track. Electricity savings grow with your utility escalation rate; gas savings grow with a separate fuel-price outlook (low / base / high), because gasoline is volatile and moves differently from regulated electricity rates.
• The "effective $/kWh on the car" readout shows what your solar is earning on those miles, so you can see the gap against your retail rate directly.

What to enter

Annual miles and your current car's real MPG; the EV's MPGe (120 is a reasonable default for an efficient sedan, lower for a truck); and a defensible gas price. If you're not sure solar will cover the extra load, watch the energy-summary strip — it shows production against household plus EV demand, and flags any shortfall that's still coming from the grid.

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