EV Charging Cost Calculator

Charging can only add energy to the battery, not remove it, so if the target charge level is at or below where the battery already sits, there is genuinely nothing to charge. The calculator clamps the charge-needed calculation to zero in that case rather than reporting a negative cost, which would incorrectly imply the calculator is modeling discharging or selling energy back.

Charging Efficiency represents energy lost as heat during the charging process, so the calculator divides the raw kWh gap between Current and Target Charge by the efficiency percentage to find the actual energy pulled from the source. At 90% efficiency, filling a 45 kWh gap actually draws 50 kWh from the wall -- the extra 5 kWh is lost, but you still pay your utility for all of it.

Yes, as long as Target Charge stays above Current Charge -- each additional percentage point of Target Charge adds the same fixed amount of battery capacity's worth of energy (divided by efficiency and multiplied by rate) to the total, so Charging Cost rises in a straight line rather than by a fixed proportion of the current cost. That's an important distinction: going from 20% to 40% (20 points) costs the same as going from 60% to 80% (also 20 points), but going from 20% to 80% (60 points) costs three times as much as either -- it's the size of the percentage-point gap that scales linearly, not the target charge value itself.

The charge gap is calculated as a percentage of total Battery Capacity, so charging from 20% to 80% on a 100 kWh battery adds 60 kWh, while the identical 20%-to-80% charge on a 50 kWh battery only adds 30 kWh. A bigger battery simply holds more total energy, so the same percentage swing represents more kWh -- and therefore more cost -- regardless of how efficient the charger is.