Guide
EV charging losses explained
Not all the electricity used during charging becomes stored battery energy. Some is used by conversion and vehicle systems, and some becomes heat. That can make meter energy greater than the battery gain without proving a fault. There is no single loss percentage you should assume for every car and session.
In plain English
Follow the energy from the house to the battery
Your house supplies alternating current, or AC. The battery stores energy supplied as direct current, or DC. During home charging, the car’s onboard charger converts AC to DC. That conversion is not perfectly efficient, and other vehicle electronics remain active while the session is running.
ADAC’s first-party charging study identifies conversion in the onboard charger and vehicle auxiliary loads as important sources of the difference. It also considers cable and battery losses. Its measured results depend on the cars, charging power and test conditions; they are not a standard percentage for UK homes.
Battery heating or cooling, where the vehicle uses it, is another reason delivered energy and stored energy may differ. Useful work done by the car during a session is not necessarily available for driving afterwards. “Losses” in a charging comparison can include that energy as well as heat from electrical conversion.
In plain English
An app and a meter may count different things
Your household meter records electricity used at home. Your charger may show the electricity passing through it, while your car may estimate the energy added to its battery. These readings measure different things, so they do not have to match.
Check what each display actually means. Great Britain’s guidance for covered smart chargepoints requires a measuring system and visibility of imported or exported electricity and charging time. That does not mean every app uses the same measurement boundary or that a chargepoint measures energy stored inside the car.
A household meter also includes other household demand. Comparing its overnight increase with the car’s gain can accidentally include heating, cooking or a home battery. Solar generation further changes how much charging energy was imported. Without separating those flows, the difference is not a clean measurement of the car’s efficiency.
In plain English
A worked example, not a typical loss figure
Hypothetical session: 42 kWh in, 40 kWh stored
For example, imagine a charge that uses 42 kWh while the battery gains 40 kWh. The difference is 2 kWh. These are example figures, not measurements of a real car or a typical loss to expect.
Of the 42 kWh used, 2 kWh was not stored: 2 ÷ 42, or about 4.8%. Compared with the 40 kWh stored, the extra electricity is 2 ÷ 40, or 5%. Both describe the same charge. When comparing percentages, check whether they refer to electricity used or energy stored.
At an assumed flat rate of 25p per kWh, 42 kWh costs £10.50. Counting only the 40 kWh stored would predict £10.00. The example deliberately leaves out other household consumption and any tariff changes. For a real bill estimate, use the energy you are paying for and your actual unit rate.
In plain English
Why the difference changes between sessions
Different onboard chargers have different conversion behaviour. Temperature, charging power and the time equipment remains active can change the result. A session starting from one battery condition should not be treated as identical to another starting somewhere else.
In ADAC’s tests, lower-power AC charging increased the proportion used by auxiliary equipment in the tested scenarios. This does not justify changing a commissioned installation limit or declaring that every car should always charge at its highest possible rate. Household capacity, charging goals and manufacturer guidance still matter.
Solar-following charging can vary in power or pause as generation changes. A short top-up can also make fixed session overhead more noticeable relative to the energy added. Do not diagnose a fault from one unusual percentage, particularly when the two readings were taken over different periods.
In plain English
Allow for losses when estimating a charge
If your charger shows the electricity used for a session, you can use that to estimate the cost. It will not include any losses between the household meter and charger. If you use only the energy added to the battery, your estimate leaves out the extra electricity used during charging.
In the charging-cost calculator, the figure you enter determines whether losses are included. The charging-time calculator uses the energy your battery needs and the lower of the car and charger’s AC power limits. It does not allow for losses or temperature, so the actual time may differ.
For a tariff with several rates, calculate the energy charged in each window separately where reliable readings allow it. Do not apply today’s cheapest rate to a whole session that extended beyond the cheap period. Installation costs and standing charges are separate questions again.
Your next step
What this means for you
- Record where each energy figure was measured and its time period.
- Keep household consumption separate where possible.
- Use repeat sessions under comparable conditions before estimating your own overhead.
- Do not copy another car’s loss percentage into a precise savings promise.
If the difference changes sharply or accompanies warning messages, ask the manufacturer or installer to check it. Follow Electrical Safety First’s advice to stop using a damaged cable. Do not investigate by opening electrical equipment or improvising measuring connections.
For your budget, use the electricity you pay for rather than only the energy the battery gains. When comparing readings, check what each one includes.
Further reading
Sources
- Last reviewed
- Charging losses in electric cars: ADAC measurement study — ADAC Technik Zentrum. German research on AC conversion, vehicle auxiliary loads and measurement boundaries. Results depend on the tested cars and conditions; UK electrical requirements differ.
- Regulations: electric vehicle smart charge points — GOV.UK / Office for Product Safety and Standards. Great Britain rules for covered private chargepoints, including default off-peak settings, randomised delay and continued charging without a communications connection.
- Electrical safety advice for electric vehicles — Electrical Safety First. Dedicated charging equipment, competent installation and stopping use of damaged charging cables.