EV & charging

EV charging losses explained

By RoadCost Editorial4 min readUpdated 27 October 2026

Charging is not perfectly efficient. Some energy is lost as heat in the charger, the cable and the battery. If you are estimating cost from battery energy, you need to account for those losses.

Quick answer

Grid energy = battery energy ÷ charging efficiency. At 90% efficiency, 40 kWh added to the battery needs about 44.4 kWh from the grid. Multiply grid energy by the price per kWh.

In this guide

1. Where the losses happen
2. A worked example
3. Avoid double counting
Questions

Key takeaways

Battery energy is not the same as grid energy.
Divide by efficiency to get grid energy.
Do not add losses twice if the meter figure already includes them.

Where the losses happen

Energy can be lost in the onboard charger, the charging cable, the battery's thermal management and the cells themselves. The exact loss depends on the equipment, temperature and charging speed.

A charger or car may report energy added to the battery. Your electricity meter reports energy drawn from the grid. The difference is the loss.

grid energy = battery energy ÷ (efficiency % ÷ 100)
energy lost = grid energy − battery energy
cost = grid energy × price per kWh

A worked example

Suppose you add 40 kWh to the battery at 90% efficiency. Grid energy = 40 ÷ 0.90 = 44.44 kWh. At 0.30 per kWh, the cost is 13.33.

If you used 40 kWh directly in the cost calculation, you would understate the cost by about 4.44 kWh.

Avoid double counting

If your charging app already shows grid energy, use that number. If it shows battery energy, apply an efficiency figure. Do not apply efficiency to a meter reading that already includes losses.

Questions

What is a normal EV charging loss?

It varies by equipment and conditions. Use the car or charger figure if available, or a conservative estimate for budgeting.

Does DC fast charging lose less?

Losses depend on the equipment and conditions. Do not assume a fixed value for every charger.

Should I use 100% efficiency?

Only if you are working from an accurate meter reading. If you are starting with battery energy, 100% will understate the cost.

Sources

US EPA — fuel economy and EV range testing — Explains MPGe and the test basis for EV efficiency.
fueleconomy.gov — understanding the EV label — How MPGe, kWh/100 miles and range appear on US labels.
US DOE Alternative Fuels Data Center — home charging — Home charging equipment, tariffs and practical considerations.
US EPA — electric vehicle myths — Common EV efficiency, emissions and charging questions.
US EIA — electricity data — US electricity prices, generation and sales data.

Try the calculator

EV charging cost calculator

Includes charging losses and optional session fees.

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A note on prices and rules

This guide explains arithmetic and units. It does not publish live fuel prices, electricity tariffs or changing tax rules. Use your own price and check official sources for anything that changes.

Page last reviewed: 27 October 2026. Formulas are covered by automated tests. Prices are inputs, not live data.