EV Charging Cost Calculator

Where the charging losses go

The gap between what your meter records and what reaches the battery has three components, and their proportions explain why Level 1 is the worst offender.

The onboard charger converts AC from the wall into DC for the battery, and that conversion is around 90 to 94 percent efficient. The loss appears as heat in the charger, which is why the front of an EV is warm after charging.

Thermal management runs while charging, conditioning the battery to its optimal temperature. In cold weather this can consume a substantial share of the energy drawn, because the pack must be warmed before it will accept charge at a reasonable rate.

Vehicle overhead is the third: the car stays partly awake for the duration, running its computers, communicating with the charger and in some models running a cabin preconditioning cycle.

That last component is roughly fixed per hour rather than per kilowatt-hour, which is exactly why Level 1 is less efficient overall. Drawing 1.4 kW instead of 7 means the same hourly overhead is spread across a fifth of the energy, so it represents five times the proportional loss. Charging slower is not charging cheaper.

Time-of-use is where the real savings are

For most EV owners the rate matters far more than the vehicle’s efficiency, and overnight charging is the largest single lever available.

Utilities offer time-of-use plans because overnight demand is cheap to serve — generation capacity sits idle and the grid is lightly loaded. Off-peak rates are frequently half the peak rate, and some EV-specific tariffs go lower still.

A vehicle that charges between midnight and six in the morning on such a plan can halve its fuel cost without changing anything else. Every modern EV can schedule charging, and most home chargers can too.

The catch is that a time-of-use plan applies to the whole house, not just the car. If your household uses a lot of electricity during peak hours, the higher peak rate can offset the charging savings. Run the arithmetic on your full bill before switching.

In some markets there are also demand charges on residential EV tariffs, which price your highest fifteen-minute draw rather than total energy. Where those exist, a lower-power charger can be cheaper than a fast one despite taking longer.

Cold weather changes everything

Winter is where EV running costs diverge most sharply from the summer figures people quote, and there are two separate effects.

Cabin heating draws directly from the battery. A combustion engine heats the cabin with waste heat it produces anyway; an EV has almost no waste heat and must generate it. Resistive heating can draw several kilowatts, and on a short commute it can rival the energy used for propulsion. Heat pump systems reduce this considerably, which is why they have become a common option.

Battery chemistry is the second effect. A cold pack has higher internal resistance and accepts and delivers energy less efficiently. It also needs conditioning before fast charging, consuming energy before any goes into storage.

Together these commonly produce a 20 to 40 percent increase in kilowatt-hours per mile in genuinely cold conditions. Preconditioning the car while it is still plugged in shifts the heating load onto the grid rather than the battery and recovers part of the difference.

If you are budgeting annual running costs in a cold climate, run this calculator twice — once with your summer efficiency figure and once with a winter one thirty percent higher — and average them.