Start with the electricity purchased, not just the battery

An EV showing 16 kWh/100 km does not automatically cost 16 times your electricity unit price to drive 100 km. The trip computer, battery and charging meter measure different things. Electricity purchased can include conversion losses and energy used while the car is plugged in, as well as energy subsequently used for driving. Check what your car's display and charging app actually report before comparing them.

Green NCAP's current home-charging metric measures grid-to-battery-output efficiency: how much electricity drawn from the grid is ultimately available at the high-voltage battery terminals for propulsion and auxiliary functions. It is broader than the efficiency of the onboard charger alone. That distinction makes it useful for understanding why a bill and a trip computer need not agree.

For a practical cost calculation, use the metered electricity attributed to the car and the price actually paid. Keep household consumption separate. At public chargers, use the session receipt rather than estimating purchased energy from the change in battery percentage.

What independent European measurements tell us

In its 2025 assessment of the CUPRA Born 60/63 e-Boost 170 kW, Green NCAP reports 89.3% grid-to-battery-output efficiency for home charging. That is a measured result for a particular European specification under its test procedure, not a universal loss allowance for every EV, charger or season. The report also judges that car's consumption display very accurate: an accurate driving display can still measure something different from purchased electricity.

For illustration only, assume a car needs 16 kWh at the battery output to travel 100 km and achieves 90% grid-to-battery-output efficiency. The corresponding grid energy is 16 ÷ 0.90 = about 17.8 kWh. Do not treat this example as a measured prediction for your vehicle. Nor should you add a loss allowance to an independent consumption figure that already includes recharging losses; first read the test's measurement definition.

Not every difference is wasted electricity. Green NCAP's November 2025 testing explanation notes that pre-warming a car while connected can use grid energy instead of draining the battery before departure. That can preserve driving range, but the energy still belongs in your electricity budget. A month with frequent preheating is not directly comparable with a mild month of otherwise similar driving.

A monthly log gives a better answer than one charging session

Choose a representative period and record the odometer and battery percentage at both ends. Starting and finishing at roughly the same charge level reduces the error from buying electricity that remains stored in the battery, or driving on energy bought before the period began. Several weeks are more useful than one short top-up, particularly when percentages are rounded.

Add the home wallbox's metered kWh and the energy on public-charging receipts, noting where each measurement comes from. A wallbox export may exclude some standby or upstream electrical consumption, so treat it as a practical usage record rather than a laboratory efficiency test. If the wallbox has no suitable meter, ask the installer about proper monitoring; the difference between two whole-house bills also includes changes in heating, cooking and other household use.

For mixed home AC and public DC charging, summing receipts is useful for budgeting, but it is not a like-for-like measurement of charging-system efficiency. The energy meters sit at different points in the supply chain. Do not apply a home AC loss percentage on top of every DC receipt.

  • Record date, odometer, starting and ending battery percentage, charging location, metered kWh and total session cost.
  • Note unusual conditions such as cold weather, roof-box use, long parked periods or repeated cabin preconditioning.
  • Purchased electricity per 100 km = total recorded charging kWh ÷ kilometres driven × 100.
  • Charging cost per 100 km = total attributable charging cost ÷ kilometres driven × 100.
  • Use comparable start and finish charge levels; label incomplete records rather than filling gaps with guesses.

Calculate a blended cost, not a headline tariff

Consider a hypothetical 1,500-km month with 270 kWh of recorded charging and similar battery levels at each end. That is 18 kWh purchased per 100 km. If all energy costs an assumed €0.24/kWh, the bill is €64.80, or €4.32/100 km. These are illustrative inputs, not current Portuguese or European average prices.

Now buy 210 kWh at that home rate and 60 kWh at an assumed public rate of €0.65/kWh. The total becomes €89.40, or €5.96/100 km, before any additional fees. The car's energy use has not changed; the charging mix has. Weight prices by the kWh purchased, not by the number of sessions: a small home top-up and a large motorway charge are not equivalent events.

Include session, time, parking or subscription charges that apply, without counting a fee twice if it is already in the receipt total. Keep charger installation, finance, insurance and depreciation in a separate ownership budget. This article calculates charging expenditure, not the complete cost of owning a car.

Compare the whole household tariff in Portugal

Energy Saving Trust's charging guidance, updated in April 2026, recommends considering time-of-use tariffs and scheduling charging during cheaper periods. Its monetary examples are UK-specific; they are not Portuguese price benchmarks. The useful principle is to match the tariff to when you can actually charge, rather than assuming every advertised overnight deal suits your household.

For mainland Portugal, ERSE's official energy-price simulator covers liberalised-market electricity offers for contracted power up to 41.4 kVA. Use your household consumption plus the EV's estimated grid demand, allocating charging to the time periods you can realistically use. Compare the total annual bill, including fixed charges and any change in daytime prices or contracted-power costs. A cheaper overnight unit rate can be offset elsewhere.

To test whether a public-charging subscription pays for itself, divide its monthly fee by the saving per kWh on eligible sessions. For example, a hypothetical €10 fee with a €0.10/kWh discount needs 100 eligible kWh in that month just to break even. Check exclusions and other fees before using that calculation; do not count charging on networks where the discount does not apply.

Owner experience is useful—but your receipts decide

In a March 2023 Škoda Storyboard interview, Switzerland-based Enyaq owner Matthias Speicher said about 90% of his charging was at home, with fast charging used for longer journeys. That is one owner's account published by the manufacturer, not an independent survey or a verified energy-cost dataset. It illustrates a possible routine, not a home-charging share that everyone in Europe can achieve.

A driver relying on public charging needs a different budget from someone with reliable overnight access. Equally, a holiday month is not a good stand-in for a whole year. Keep separate records for ordinary weeks and long trips, then weight them by your actual driving pattern.

Sources were checked on 31 August 2026. The efficiency evidence is measured, the owner example is anecdotal, and all euro calculations here are explicitly hypothetical. The practical next step is a complete charging log: establish your purchased kWh/100 km and blended cost first, then decide whether changing the tariff or charging routine would genuinely save money.