Why a 36 kWh battery top-up can use more at the socket
The battery increase and the electricity purchased are not always the same number. State the boundary before comparing prices.
Put it into practice
Try the tool EV Charging Cost · Free, with local processing.
The short answer
The battery increase and the electricity purchased are not always the same number. State the boundary before comparing prices.
The EV Charging Cost helps you turn the inputs into an estimate you can inspect. The useful part is understanding which assumption drives the answer, rather than treating the largest number on screen as a decision by itself.
Work through an example
Charging a usable 60 kWh battery from 20% to 80% adds 36 kWh. At 90% efficiency, modeled wall energy is 40 kWh. Prices of 0.25 and 0.55 give session estimates of 10 and 22.
The example uses explicit inputs so you can reproduce it. Replace them with your own measurements or assumptions, and keep the units consistent. A result can be arithmetically correct while answering the wrong question if the inputs describe something else.
Use this checklist
- Use usable capacity and the intended charge window, not the total advertised battery size blindly.
- Choose an efficiency assumption and keep battery-side and wall-side energy distinct.
- Check whether a public quote also includes time or session fees.
How the calculation works
Battery energy added = usable capacity × (target charge − starting charge) ÷ 100. Wall energy = battery energy ÷ efficiency fraction. Each modeled cost = wall energy × its price per kWh.
The tool shows its result and supporting values together. Where an item must be bought or counted in whole units, read the rounded count as well as the underlying quantity. Display rounding is separate from real-world uncertainty.
The mistake to avoid
Do not add losses to a metered wall-energy total again. Public tariffs may include time, idle or session fees. Battery capacity, temperature and charging behavior change real consumption.
Compare at least one realistic alternative input before making an important purchase or plan. If the result looks surprising, inspect the units, the source of each input, and what the calculation leaves out first.
A useful next step
Open the EV Charging Cost to try the example. You can also explore Trip Fuel Cost & Split for the next part of the workflow. The related tool is optional; using one estimate does not mean you need to make a purchase or change your routine.
Method and scope
This article explains the arithmetic implemented in the tool. It does not establish a supplier specification, institution policy, provider fee schedule, or individual professional recommendation.
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