An EV charging bill is easier to understand when you separate three ideas: how much energy the battery receives, how much energy comes from the wall, and how far the vehicle travels on that stored energy.
Battery capacity is measured in kWh
An EV battery might be described as 60 kWh, 75 kWh, or 100 kWh. That number is an energy quantity, not a charging speed. A larger battery can store more energy, but it does not automatically charge faster.
Charging power is measured in kW
A home Level 2 charger might deliver around 7 kW or more, depending on the circuit, charger, and vehicle. Charging time depends on both the energy needed and the power available.
Approximate time = energy needed ÷ charging power
Real charging time can be longer because charging power changes and because some energy is lost.
The wall supplies more energy than the battery stores
Charging is not perfectly efficient. Energy is used by power electronics, battery conditioning, cooling or heating, and other systems. If a battery gains 45 kWh, the utility meter may record more than 45 kWh.
Our EV calculator models this with a charging-efficiency input. At 90% efficiency, putting 45 kWh into the battery requires about 50 kWh from the wall.
Cost comes from wall energy
Your utility bills the electricity drawn from the grid, so charging cost should be based on wall energy, not only the increase in battery energy.
Miles per kWh connects electricity to driving
Vehicle efficiency is often expressed as miles per kWh. A car averaging 3.5 miles/kWh uses about 28.6 kWh of battery energy to travel 100 miles. After charging losses, wall energy will be somewhat higher.
Why home charging can vary by time of day
If your utility has time-of-use rates, charging at night may cost less than charging during peak afternoon or evening hours. A single statewide average cannot capture that. Enter your actual off-peak or on-peak rate when comparing charging schedules.
Public charging is a different market
Public chargers may price by kWh, time, session, membership plan, or other structures, and their prices can be much higher than residential electricity. A home-charging calculator should not be assumed to represent public fast-charging cost.
Battery size does not determine monthly cost by itself
A 100-kWh battery does not cost more every month merely because it is large. Monthly energy cost depends primarily on miles driven, vehicle efficiency, charging losses, and electricity price. Battery capacity matters most when estimating the cost of a particular charging session or a full recharge.
Further reading
For general electricity units, see EIA's Measuring electricity. For practical cost calculations, use our EV charging cost calculator and EV charging time calculator.