The simple version: every kWh of sunshine ampee puts into your car instead of selling it to the grid saves you the difference between the grid price you'd otherwise pay and the small credit you'd get for exporting it. On typical California rates that's about 34¢/kWh to buy vs ~3¢/kWh to sell — so roughly 31¢ saved for every kWh of sun you self-consume. (We value your solar at that gap, not the full retail price, because exporting it would still have earned you the few-cent credit — so this is the true extra you keep.)

A fixed amp setting can't react to the sun, so it loses money two ways:

• It wastes solar (green band above). When your panels are producing more than your fixed setting draws, the extra gets sold back to the grid for pennies (about 3¢/kWh) instead of going into your car.
• It pulls from the grid (amber band above). When clouds or your house's own usage cut into what's left for the car, a fixed setting keeps drawing the same amps regardless — the shortfall comes from the grid, often at daytime rates.

ampee adjusts your charge rate every few minutes to track actual solar output, so it captures the first kind of waste and avoids the second. Running the AC, a pool pump, or a water heater at the same time makes the second kind worse — every time one cycles on, it eats into what's left for the car, and a fixed setting can't react. Drag across the chart above to see those two effects turn into dollars, hour by hour.

Each vehicle's charging need is calculated from its own weekly mileage at a flat ~300 Wh/mile (a typical EV average — actual efficiency varies by model, roughly 250-450 Wh/mile), spread across the days/week you said that car is home — vehicles with different schedules get different results. Uses the solar system size you set above (the suggested default grows with EV count; set your real size for a better estimate), a single charger shared across all vehicles, charging starting around 8am, and California E-ELEC time-of-use rates ($0.37/kWh daytime, $0.31/kWh overnight, $0.03/kWh solar export). If you have a home battery, we assume it's about 20% charged at sunrise, charges at up to 5kW, and always charges first — neither charging strategy draws anything, from solar or the grid, until the battery reaches full, and it never discharges into the car. A bigger battery delays both equally; it just leaves less of the day for whichever strategy you're using to make its case. Your real numbers depend on your rate plan, vehicles, and battery's own settings — this is an estimate, not a quote.