The useful starting point
To add 36 kWh to a battery with 10% charging losses, you buy 40 kWh—not 39.6 kWh. At 30p/kWh, that is £12. The right loss allowance depends on how and where you charge.
Where does the extra electricity go?
The energy recorded by a charger or electricity meter is not necessarily the energy stored in the battery. Power conversion, electrical resistance and equipment running during the session all use energy. Heating or cooling the battery can add to that demand.
For AC charging, the car converts alternating current to the direct current its battery stores. For DC charging, that conversion happens in the charging equipment. This distinction matters when comparing a home meter reading with a public charging receipt: they may measure energy at different points.
Here, “losses” is a budgeting term for that difference. Battery heating can be useful even though it does not increase stored energy. The allowance is not a measurement of battery degradation.
What loss percentage should I use?
Use your own comparable charging sessions when you have them. Otherwise, treat the calculator’s loss setting as an assumption and compare a lower and higher value. There is no single percentage that is accurate for every EV, power level and temperature.
ADAC’s controlled AC tests of five cars found losses of 5.1–7.0% at 11 kW, compared with 12.7–24.2% at a 2.3 kW socket. The batteries were at 20–30°C, with measurements inside the 10–90% charge window. Those figures do not establish a default for a UK 7.4 kW wallbox, winter charging or every vehicle.
- Home wallbox: check sessions at your usual power. A load limit or solar-surplus setting can change the conditions, so keep those sessions separate.
- Low-power charging: a longer session keeps the car’s systems running for longer. A percentage taken from a faster session may understate the energy used.
- Rapid DC: use the energy shown on the receipt as your starting boundary. Do not automatically add the charger’s upstream conversion losses to energy already billed at its output.
- Cold or very hot conditions: allow for battery conditioning. Preconditioning before arrival moves some energy use earlier; it does not make that energy free.
If the available evidence is weak, label the result as a range. A planning estimate of “about £12–£13” can be more honest than a precise-looking figure based on an untested allowance.
How much do charging losses change the cost?
Keep the amount reaching the battery constant when comparing assumptions. This example uses a 60 kWh usable battery, 20% to 80%, and 30p/kWh. It adds 36 kWh to the battery in every row. Rates and loss values are illustrative, not tariff quotes or recommended settings.
| Loss allowance | Energy supplied | Energy cost |
|---|---|---|
| 0% | 36.00 kWh | £10.80 |
| 5% | 37.89 kWh | £11.37 |
| 10% | 40.00 kWh | £12.00 |
| 15% | 42.35 kWh | £12.71 |
| 20% | 45.00 kWh | £13.50 |
The 0% row is a mathematical baseline. At 10% losses, the bill is £1.20 higher than that baseline; at 20%, it is £2.70 higher. Session fees, parking and other charges are not included in this table.
Why not multiply by 1.10? That would add 10% of the battery energy. EV Calculated defines losses as a share of supplied energy: 4 of the 40 kWh, or 10%. These are different denominators.
Check the estimate against a real session
- Record a consistent set of readings. Note the car’s starting and ending charge percentage, usable battery capacity, energy delivered, charging power and conditions. Use a charger’s session reading or a dedicated meter where possible. A whole-house reading includes other appliances.
- Estimate the stored energy. Multiply usable capacity by the percentage-point increase divided by 100. A 60 kWh battery rising from 20% to 80% stores approximately 36 kWh.
- Compare like with like. If the session meter records 40 kWh, the implied loss is (40 − 36) ÷ 40 × 100 = 10%. Include only the energy boundary you intend to pay for or compare.
- Repeat under similar conditions. Compare several normal sessions before changing your planning assumption. Keep wallbox, low-power and rapid sessions separate.
- Update the relevant setting. In the full calculator, open Settings and adjust the charging-loss allowance for that charging type. Recheck the electricity rate before using the result as a budget.
This is an estimate, not a battery health test. The displayed charge percentage is rounded; usable capacity may be an estimate and can change with battery condition. Small top-ups magnify these uncertainties. A negative or unexpectedly large result is a reason to check the inputs and measurement boundary, not proof of a fault.
Avoid counting the same loss twice
A car’s displayed driving efficiency and an electricity bill can describe different energy boundaries. If your efficiency figure already comes from metered electricity divided by distance, it already includes charging losses. Applying the same loss allowance again overstates the cost.
EV Calculated’s efficiency input is based on battery energy: for example, 4 miles per battery kWh. At 30p/kWh and 10% losses, electricity costs 30 ÷ (4 × 0.90) = 8.33p per mile. Without the allowance, it would be 7.50p per mile. See the fuel comparison method for the full calculation.
Likewise, use the price and energy quantity from the same billing boundary when assessing public charging. A study’s total grid-to-battery loss is not automatically an extra percentage to add to a public charging receipt.
Questions that matter in practice
Does a lower electricity rate always make slower charging cheaper?
No. Compare the total supplied energy at each rate. For the same 36 kWh battery gain, 10% losses at 30p/kWh costs £12. At 20% losses and 20p/kWh, it costs £9. The cheaper rate wins in this example, even with greater losses. With closer rates, the answer can change. Use the full charging comparison with independent assumptions for each option.
Should I use the same allowance all year?
Only if it remains a useful approximation for your sessions. Temperature, charging power and battery conditioning can change the result. Use representative warm- and cold-weather observations if seasonal budgeting matters to you.
Are charging losses the same as losing range while parked?
No. This guide concerns energy used during charging. Energy consumed while parked after a session is a different part of your overall electricity budget and is not separately modelled by the charging-loss setting.
Sources & scope
Sources checked on 3 November 2026. Worked examples use the stated illustrative inputs; they are not measurements made by EV Calculated.
- ADAC: measured AC and DC charging losses
Original test report (German). Different vehicles, charging powers and measurement boundaries; results are not a universal loss allowance.