You plug in with 40% showing on the battery. A few hours later, the car reads 80%, yet the electricity drawn from the supply looks higher than a quick calculation based on battery capacity. What looks like energy lost when charging an electric car has not slipped into an electrical Bermuda Triangle. The two figures are simply measuring different things.
The supply meter records electricity drawn from the grid. The charging equipment carries and controls the AC supply to the car. Once that energy enters the vehicle, the vehicle decides where it goes: some is converted and stored in the traction battery, while some may power control units, low-voltage systems, battery heating or cooling, and even cabin climate control if it is running during the session.
That means the difference between the supply reading and the energy added to the battery is not automatically a loss caused by the charging equipment. Several stages are involved, and they are not all controlled by the same device. A plugged-in laptop offers a useful comparison: mains electricity can run the screen, processor and fan while also charging the battery. Looking only at the battery increase will not reveal everything the laptop used.
EV charging efficiency depends on the vehicle, temperature, charging power and, most importantly, where the measurement begins and ends. The resulting EV charging losses cannot be reduced to one percentage that applies to every car.
Compare the right figures before calculating a loss
Three measurements may appear during the same charging session:
- electricity drawn from the grid, recorded by the home meter or charge point;
- electricity delivered by the charging equipment;
- electricity actually added to the traction battery.
These figures are related, but they do not cover the same boundary. Between the grid and the output of the charging equipment is the part that the equipment can monitor or control. Once the electricity enters the car, AC-to-DC conversion, vehicle-system consumption and battery storage begin. That allocation depends on the vehicle and the conditions of the session, not on the external AC charging equipment.
A whole-home meter may also be counting the immersion heater, oven or space heating. Meanwhile, a battery percentage is not a kilowatt-hour meter. It depends on usable battery capacity and on how the vehicle estimates its state of charge.
Take a simple example. Moving from 40% to 80% on a battery advertised as 60 kWh does not prove that exactly 24 kWh was stored during that session. Advertised capacity may differ from usable capacity, which can also be affected by temperature, battery age and the protection margins selected by the manufacturer. The calculation is a useful estimate, not a laboratory measurement.
For a fair comparison, use the same measurement point, the same car and a similar state-of-charge window. Otherwise, it is rather like comparing an apple, a pear and a dashboard display that is politely trying to simplify both.
Where does the energy go during AC charging?
At home and at an AC charge point, the supply provides alternating current, while the battery stores direct current. The car's onboard charger therefore converts AC into DC. That conversion is not perfectly efficient, so some energy becomes heat.
The car is not entirely asleep during charging either. Control units, contactors and low-voltage systems remain active to manage the session. Depending on the vehicle and the temperature, thermal management may heat or cool the battery. The circuit, connections and cable also have electrical resistance, although their contribution is generally smaller when the installation and connections are suitable.
| Part of the charging chain | What happens there | Mainly controlled by | When it may be more noticeable | What the driver can check |
|---|---|---|---|---|
| Circuit and connections | Electrical resistance turns a small amount of energy into heat. | Electrical installation and charging equipment. | A long or ageing circuit, unsuitable connection or unusual heating. | Socket condition, circuit suitability and a professional assessment if anything is uncertain. |
| Onboard charger | It converts AC electricity into the DC electricity required by the battery. | Vehicle. | Conversion efficiency varies by vehicle and operating point. | The car's accepted AC power and the conditions used in any reference test. |
| Vehicle electronics | Control units and supporting systems remain powered during charging. | Vehicle. | A long session at lower power, because these systems remain active for longer. | Session duration and settings recommended by the vehicle manufacturer. |
| Thermal management | The vehicle may heat or cool the battery. | Vehicle. | A cold or hot battery, rapid charging or more demanding weather. | Ambient and battery temperature, preconditioning and information supplied by the car. |
| Battery | Electrochemical storage also produces some heat. | Vehicle and battery-management system. | Depending on temperature, state of charge, battery chemistry and vehicle strategy. | Compare several sessions over similar charge windows rather than relying on one isolated result. |
This table describes an energy chain, not five fixed percentages to add together. It also shows why the entire difference should not be blamed on the cable or charging equipment. Once electricity passes into the car, the vehicle decides how it is used. The contribution of each stage changes with the model and the session.
Why very slow charging can show higher losses
Some energy use continues for as long as the charging session lasts. If the vehicle electronics remain active for one hour, their share of the total may be modest. If the same amount of battery energy takes all night to add, those systems operate for much longer and can account for a greater amount of the electricity drawn.
ADAC tested five vehicles during AC charging from a 2.3 kW socket, from an 11 or 22 kW wallbox at its maximum power, and from a wallbox limited to 4.1 kW to simulate lower-power solar charging. The tests covered a 10% to 90% state-of-charge window, with battery temperature between 20 and 30°C at the beginning of each test.
Under that specific protocol, all five vehicles recorded their highest losses at 2.3 kW and their lowest at 11 or 22 kW, although the results varied considerably between models. The study illustrates a mechanism: when auxiliary systems remain active for longer, their consumption carries more weight. It does not turn maximum supported power into a universal instruction.
The car, supply point, circuit and protective equipment determine the power that is actually suitable. Compatibility and installation quality matter more than chasing a few percentage points. Our guide to choosing a home EV charger size in Ireland treats that decision separately.
Temperature changes the result too
A battery performs best within a particular temperature range. When it is cold or very hot, the vehicle may use energy to prepare it for charging or keep it within a suitable range. That energy appears on the supply side but does not directly increase the amount stored in the battery.
The effect depends heavily on the situation. During a short session in cold weather, the initial energy needed for battery heating can represent a noticeable part of a small total. Over a longer session, that initial demand may account for a smaller share. Rapid charging, on the other hand, may require cooling. There is no single winter correction that can be applied to every car.
One unusual session is not enough to diagnose a fault. Several readings taken at the same point, across similar charge windows and temperatures, usually tell a much more useful story.
AC and DC: the converter changes location
During AC charging, conversion to DC happens inside the car. During DC charging, it happens in the charging station before the electricity reaches the vehicle. Some conversion loss therefore sits upstream of the DC cable and may not appear in the same way on every display.
The vehicle can still use energy for its own systems and, where needed, battery heating or cooling. Describing DC charging as loss-free would therefore be misleading. Any comparison between home AC charging and DC rapid charging needs to state where the electricity was measured and which equipment was included.
How to make a more meaningful comparison
Start by identifying what each figure measures. A dedicated charging meter is easier to match to one session than a monthly reading for the entire home. If the vehicle provides a clearly defined value in kilowatt-hours, that is usually more informative than battery percentage alone.
It also helps to compare several similar sessions rather than drawing a conclusion from one night. Temperature, charge window, duration and power setting provide useful context. A sudden change accompanied by unusual heating, a damaged socket or repeated errors deserves an assessment by a qualified electrical professional.
The aim is not to pursue an impossible 100%. It is to separate normal operation in the charging chain from a poorly framed comparison or a genuine problem.
One portable system, different supply conditions
For Irish drivers, the Rheidon PC200 Pro-11K provides a useful example of why the supply connection matters. On a suitable three-phase CEE 16 A connection, and with a compatible vehicle, it can deliver up to 11 kW. With the separately sold Type G 13 A Quick-Switch adapter, intended for the three-pin socket format used in Ireland, the listed maximum is 3.0 kW.
Those figures describe two different supply conditions, not two efficiency promises. The Quick-Switch system allows the charger to recognise the compatible adapter and set the available range accordingly, while the circuit and the car still determine what can actually be used. Once the energy enters the vehicle, the PC200 Pro does not decide how much goes into battery storage, thermal management or other active systems.
Frequently asked questions
Is the entire difference between the meter and the car display a loss?
Not necessarily. Both readings need to cover the same session and measure comparable quantities. A whole-home meter includes other household loads, while battery percentage is not a direct measurement of kilowatt-hours stored.
Even with a meter dedicated to charging, some of the electricity entering the vehicle may power control units, low-voltage systems, thermal management or cabin climate control. The vehicle has used that energy, but it has not all been stored in the traction battery.
Does slow charging always waste more electricity?
No. A longer session can increase the energy used by systems that remain active throughout charging, but the result still depends on the car, temperature and the operating point of its onboard charger. The selected power must also suit the circuit, charging equipment and vehicle guidance.
Can a portable EV charger eliminate EV charging losses?
No. AC-to-DC conversion, active vehicle electronics and battery storage all use or dissipate some energy regardless of the AC charging equipment. Suitable equipment provides a compatible, controlled charging connection; it cannot guarantee the same efficiency for every car.
Are DC rapid-charging losses the same?
No. The main AC-to-DC conversion takes place in the charging station. The vehicle may still use energy to heat or cool its battery, however, so the measurement boundary must be understood before AC and DC figures are compared.
How can I tell whether a published loss figure applies to my car?
Look for the vehicle tested, charging power, temperature, state-of-charge window and meter location. If those details are missing, the percentage may illustrate a general point but cannot reliably predict one of your own charging sessions.
EV charging losses are not automatically evidence of a fault, nor are they one hidden fixed number shared by every car. They describe several stages between the grid and the battery, handled in turn by the installation, charging equipment and vehicle. Once those boundaries are clear, the gap becomes much less mysterious and different tests can finally be compared on the same terms.
Sources
- ADAC, “Ladeverluste beim Elektroauto: Wie hoch sind sie wirklich?”, 2026. The cited findings remain tied to the vehicles, power levels, temperatures and measurement boundaries in that test protocol.
- U.S. Department of Energy / NREL, Plug-In Electric Vehicle Handbook for Electrical Contractors, used for the general role of AC charging equipment and the onboard charger. Its US electrical categories are not presented as guidance for Ireland.
