Calculate the electricity used for each EV charging session
Vehicle charging can be priced by the kilowatt-hour, by time, or through a combination of electricity charges and service fees. The battery energy added is not necessarily the same as the electricity delivered by the charging equipment. Some input energy is lost as heat in the conversion and charging process, and temperature management can also consume power. For an electricity-cost estimate, the relevant quantity is the energy drawn from the supply, not only the energy stored in the battery. Enter the electricity price from the applicable household tariff or charging provider rather than using an assumed national rate.
Charging efficiency is entered as a percentage of wall energy ultimately stored in the battery. The selected average charging power determines approximate session duration; it should represent the power sustained over the session, not an advertised peak that the vehicle cannot maintain. Public charging fees and parking charges are counted separately from electricity so their effect is visible. A monthly subscription is counted once, while a per-session fee repeats with each visit.
Charging energy, duration and monthly cost formulas
Energy lost (kWh) = electricity drawn − battery energy added
Charging duration (hours) = electricity drawn ÷ average AC power (kW)
Monthly cost = electricity drawn × sessions × $/kWh + per-session fees × sessions + monthly membership
Worked example: twelve charging sessions per month
Suppose each session adds 35 kWh to the battery, with a modeled wall-to-battery efficiency of 90%. The outlet supplies approximately 38.89 kWh per session, including about 3.89 kWh of losses. At an average supply power of 7.2 kW, the idealized session duration is approximately 5.40 hours. At twelve sessions per month, the vehicle draws approximately 466.67 kWh from the supply.
If the energy rate is $0.20 per kWh and there are no additional fees, modeled monthly charging electricity costs approximately $93.33. A charging-network fee of $2 for each session would add $24 to that month; it is not part of the electricity rate.
How home electricity tariffs differ from public charging bills
A home's effective charging price may depend on time-of-use billing, seasonal rates, fixed delivery charges and utility demand charges. This calculation applies one variable energy price to all of the modeled sessions. To compare peak and off-peak scenarios, change the rate and record the resulting costs separately. Fixed household charges that would exist without the vehicle are not automatically attributable to EV charging, and any charger installation cost should be estimated separately.
Public chargers can also impose flat connection charges, idle-time penalties or subscription arrangements. A time-billed charger cannot be modeled accurately from a flat kWh rate without determining its charging duration and provider terms. The displayed running time is useful for planning, but battery taper, preconditioning, charging interruptions, and station power sharing can all change the elapsed time. Treat the result as an energy and billing scenario rather than a charging-station quote.
EV charging cost questions
Why is electricity drawn greater than energy delivered to the battery?
Charging equipment and the vehicle's onboard systems use some of the incoming electricity before it is stored. Dividing delivered energy by a charging efficiency below 100% models the extra electricity purchased. Real efficiency varies with power level, temperature, state of charge and equipment, so a measured charger energy reading is preferable when available.
Does the result include installing a home EV charger?
No. The calculation describes recurring charging electricity and optional service fees. A home charger installation involves circuit routing, protective devices, equipment selection and electrician labor. Those costs belong in a separate installation quotation rather than being mixed into each charging session.
Can I compare two different charging networks?
Yes, by entering each network's rate, subscription and per-session fees in separate scenarios. Check whether the provider bills by kWh or elapsed minutes and whether taxes, idle charges or minimum session charges apply. A per-minute tariff requires particular attention to sustained charging power and cannot be reproduced faithfully using an energy-only rate.