The energy stored in an EV battery is not identical to the energy drawn from the wall. Charging efficiency captures the difference, helping you estimate both cost and the electricity demand on the circuit.
Where charging energy goes
Losses can occur in the onboard charger, wiring, battery conditioning, cooling, and the conversion between alternating and direct current. The exact share changes with charger type, temperature, battery state, and charging power. Treat efficiency as an assumption to test, not a universal constant.
Worked example
If the battery accepts 45 kWh and the charging efficiency is 90%, the wall supplies 45 ÷ 0.90 = 50 kWh. The difference is 5 kWh. At $0.16 per kWh, that loss represents $0.80 for the session.
How to choose an assumption
Use measured energy from a charger or utility monitor when available. Otherwise calculate a conservative and optimistic range, such as 85% and 95%, and show how the result changes. A single precise-looking efficiency number can hide meaningful uncertainty.
Avoiding a planning mistake
Do not size a circuit from battery energy alone. Charger output, available hours, electrical headroom, and local code requirements also matter. Use the charger-sizing tool and a qualified electrician for installation decisions.
Sources
The figures and rules in this guide are checked against:
- Charging Electric Vehicles at Home — U.S. DOE Alternative Fuels Data Center
- Plug-in Electric Vehicle Charging: The Basics — U.S. Environmental Protection Agency
- Electric Vehicles: Learn More About the New Label (kWh/100 miles) — fueleconomy.gov (U.S. DOE / EPA)
- Charging — Tesla Support