Practical guide · stated assumptions and checkable examples
EV charging costs: battery energy is not always billed energy
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Source and worked-example review; not professional advice or approval. Examples are synthetic unless explicitly identified otherwise. Follow the cited authority for current eligibility and legal requirements.
The battery receives energy, while the electricity meter records energy supplied to the charging process. A cost comparison should say which of those quantities it uses and should not silently charge for the battery’s entire capacity every time the vehicle is plugged in.
Calculate the energy added to the battery
For a synthetic 60 kWh usable battery charged from 20% to 80%, the change is 60% of capacity: 36 kWh. The state-of-charge difference is a percentage-point change, not a 60% increase relative to the starting charge. Use the capacity appropriate to the scenario rather than assuming every vehicle’s nameplate capacity is fully usable.
Calculation: Battery energy added = usable capacity × (ending charge − starting charge) / 100
Apply an explicit efficiency assumption
At an illustrative overall charging efficiency of 90%, wall energy is 36 / 0.90 = 40 kWh. At an invented rate of 0.20 per kWh, energy cost is 8.00. The assumed 90% is not a verified specification for a vehicle or charger. DOE treats charging and battery systems as distinct technical components; actual energy and power depend on the equipment and operating conditions.
References: US Department of Energy — Batteries, charging and electric vehicles
Compare distance costs on equal boundaries
If a vehicle’s battery-side consumption is entered as 18 kWh per 100 km, the same 90% assumption implies 20 kWh from the wall and 4.00 of energy cost per 100 km at the example tariff. A petrol scenario at 7 L/100 km and 1.50/L gives 10.50 per 100 km. These invented input pairs compare energy cost only, not purchase price, maintenance, insurance or depreciation.
Time needs a power assumption as well
At a constant 7 kW wall supply, 40 kWh takes 5.714 hours. This is an idealized average-power calculation. It excludes changes in available power, tapering, preconditioning and interruptions. Do not use kW and kWh interchangeably: power multiplied by hours gives energy. NIST’s unit framework supports that distinction, not a guaranteed completion time.
References: NIST — Guide for the Use of the International System of Units
Read the tariff before annualizing
Add any per-session, parking, subscription or time-based charges separately. A household standing charge that would exist without the vehicle is different from a new subscription taken specifically for charging. For time-of-use pricing, split energy into the applicable periods rather than assigning the cheapest unit rate to every charge. Save both the date and the source of each price used.
Questions about this guide
Should I multiply by 90% to get wall energy?
No. When efficiency is battery energy divided by wall energy, divide the battery energy by 0.90 to calculate the larger wall-energy amount.
Are the charging rate and vehicle consumption live data?
No. They are entered scenario values. Verify vehicle, charger and tariff details before using the estimate for a budget.