Audit an EV charging bill: from battery percentage to cost per kilometre
A dashboard figure, a charger receipt and a household electricity bill can all be correct while showing different numbers. This guide follows one hypothetical charging session through those boundaries, then builds a monthly comparison that can be repeated with your own records.
Written for WattCalc Lab by Tomas Baranauskas. The figures below are deliberately constructed examples, not a vehicle test, a customer bill or current market prices.
1. Decide which energy you are comparing
Battery energy is what is stored or used inside the vehicle. Meter energy is what crosses the selected billing meter. A vehicle trip display usually describes driving consumption; a dedicated charger meter may also include conversion losses and energy used during charging. The household meter additionally records unrelated household loads. A change in the whole-house meter is therefore not automatically an EV charging measurement.
Choose one question before collecting data: the price of one session, the cost of driving a kilometre, or the cost of switching tariff. Use the same boundary for both sides of a comparison. Do not compare battery-side kWh with billed petrol costs while ignoring electricity needed to replace charging losses.
2. Reconcile one charging session
| Record | Value | Meaning |
|---|---|---|
| Usable battery capacity | 60 kWh | Assumed accessible capacity, not a gross pack label |
| Start → finish | 20% → 80% | A 60 percentage-point increase |
| Stored energy estimate | 36 kWh | 60 × (80 − 20) / 100 |
| Dedicated meter | 42 kWh | Input energy recorded for this session |
| Implied session efficiency | 85.7% | 36 / 42 × 100 |
| Energy tariff | €0.25/kWh | 42 × 0.25 = €10.50 |
| Separate session fee | €1.50 | Total receipt €12.00 |
The 6 kWh gap does not by itself show a faulty meter. It reflects the different boundaries and the uncertainty of estimating stored energy from capacity and rounded percentages. Do not report the implied 85.7% as a certified efficiency for that model. Repeat comparable sessions and note preconditioning, charging power and temperature before deciding whether the estimate is useful.
To reproduce the energy charge in EV charging cost, enter 60 kWh, 20%, 80%, €0.25/kWh and approximately 85.714% efficiency. The tool should give approximately €10.50 before fees. Add the €1.50 fee separately. Default inputs and currency settings do not update this fixed example.
3. Convert a period of records to driving cost
Suppose a recording period contains 600 km and 132 kWh bought for charging. Start and finish battery charge must be reasonably similar; otherwise a partly filled battery at the end leaves energy paid for but not yet driven. With aligned boundaries, 132 / 600 × 100 gives 22 kWh/100 km at the meter. At €0.25/kWh the energy cost is €5.50/100 km. If total session fees during that period are €6, they add €1/100 km: total charging cost becomes €6.50/100 km.
The EV versus fuel calculator accepts meter-side EV consumption directly. Enter 22, not a lower battery-display rate, for this example. It has no separate efficiency input and does not include the €6 fees. The charging-cost calculator does have an efficiency input: when feeding it an already meter-side per-distance rate, use 100% for that per-distance comparison to avoid adding losses again.
4. Weight mixed tariffs by energy
Suppose 100 of the 132 kWh cost €0.20/kWh at home and 32 cost €0.60/kWh in public. Energy spending is €20 + €19.20 = €39.20. The correct weighted price is 39.20 / 132 = €0.297/kWh. Averaging the two tariffs as (0.20 + 0.60) / 2 would produce €0.40/kWh and overstate this example’s energy bill by €13.60. The number of sessions is irrelevant to the energy weighting.
Subscriptions require a different check. A €10 monthly fee that reduces a public tariff by €0.10/kWh breaks even at 100 kWh/month at that network, before other differences. The discounted tariff alone is insufficient. Count only the kWh eligible for that discount and keep parking and idle fees separate.
5. Keep a repeatable record
- For each session, record the date, charger or meter boundary, start and finish percentages, billed kWh, energy price and separate fees.
- For each comparison period, record odometer start and finish and battery charge at both ends. Note charging that belongs outside the period.
- Record the source of capacity: a source-backed usable figure or a value supplied and checked by you. Model selection is not proof of every specification.
- Keep unusual preheating, towing, motorway travel and weather in the notes. Analyse a normal commute separately from a holiday trip.
- Sum costs and kWh first; divide afterwards. Keep unrounded arithmetic in your record and round only the displayed result.
6. Use the audit for a decision
If the bill matches the ledger, the next question is which part you can change. A high energy tariff suggests testing a different tariff mix. A large fee per kWh suggests fewer small public sessions where practical. A low implied efficiency calls for comparable measurements before blaming equipment. A cheap overnight tariff is useful only if the cheap window can actually deliver the required energy at the permitted charging power.
This audit does not establish battery health, diagnose a charger or predict annual ownership cost. Its purpose is to make the accounting visible so that a decision follows your records rather than a catalogue default.
Reference and scope
EIA: measuring electricity explains the distinction between power and energy units. Accessed 11 October 2026. The ledger, tariff weighting and break-even calculations above are WattCalc Lab’s own arithmetic examples; the reference does not validate the vehicle assumptions.