You pay for energy at the meter, not energy in the battery
The meter — whether it is your utility meter or the one inside a public charger — counts every kilowatt-hour that passes through it. Some of that never reaches the battery. Alternating-current charging loses energy in the onboard rectifier that converts AC to DC, in resistive heating in the cable and connector, and in the battery's own internal resistance. Direct-current fast charging skips the onboard converter, so it is generally more efficient at the vehicle end, though the cabinet itself has losses of its own that are already inside the price you are quoted.
The practical consequence is one arithmetic step: divide by efficiency, do not multiply. If you want 45 kWh in the battery at 90% efficiency, you buy 45 ÷ 0.90 = 50 kWh. Multiplying instead gives 40.5 kWh and understates the bill by about a fifth of the loss. Over a year of home charging that error compounds into a real number.
Efficiency is not a fixed property either. AC charging at a low power level pays the same fixed overheads — the onboard charger's own electronics, the battery management system, the coolant pumps — over fewer delivered kilowatt-hours, so a slow Level 1 charge on a household outlet is meaningfully less efficient than a Level 2 charge. Cold weather is worse again, because the pack may be heated during charging and that heat comes out of the same meter reading.
Then there are fees. Many public networks charge a flat session fee and nearly all now charge an idle fee once your car has finished but is still occupying the stall. Those are not energy costs and they do not scale with kilowatt-hours, so on a small top-up they can easily exceed the electricity itself.
Working through the calculation
Start with what you want in the battery. A state-of-charge change of ΔSOC on a pack with usable capacity C is C × ΔSOC / 100 kilowatt-hours. Use the usable capacity, not the gross pack size — manufacturers reserve a buffer at the top and bottom that the dashboard percentage never touches, and a 77 kWh gross pack may have 75 kWh usable.
Divide by charging efficiency to get billed energy, multiply by your rate, then add the flat fees. That is the session cost.
The per-mile figure needs one more number: how far the car goes on a kilowatt-hour out of the battery. Range added is energy into the battery × mi/kWh, and cost per mile is the session cost divided by that. Note that the losses appear in the numerator, not the denominator — you paid for them, but they never became range. That is exactly why a charging-cost calculation that ignores efficiency understates cost per mile.
This is where the EPA convention differs from the trip computer. The EPA fuel economy label reports electric consumption as kWh per 100 miles measured from the wall, so charging losses are already inside it. Your car's trip computer normally reports mi/kWh measured from the battery. Both are correct; they just count different things. Feed this calculator a battery-side figure from the trip computer and let it apply the efficiency, or feed it a wall-side figure and set efficiency to 100% — but do not do both, or you will apply the losses twice.
Time-of-use rates deserve a separate run. Enter your off-peak rate for the overnight charging you actually schedule, then use the comparison rate field for your peak rate, and the annual difference tells you what the scheduling discipline is worth.
Worked example: a 75 kWh car from 20% to 80% at home
A 75 kWh usable pack goes from 20% to 80% overnight on a Level 2 charger at a residential all-in rate of $0.16 per kWh. Charging efficiency is 90% and the trip computer averages 3.5 miles per kWh.
- State of charge added. 80 − 20 = 60%.
- Energy into the battery. 75 × 0.60 = 45.0 kWh.
- Energy billed. 45.0 ÷ 0.90 = 50.0 kWh. The 5 kWh difference is the loss you pay for and never use.
- Energy charge. 50.0 × $0.16 = $8.00. No session or idle fee at home, so that is the session total.
- Range added. 45.0 × 3.5 = 157.5 miles.
- Cost per mile. $8.00 ÷ 157.5 = $0.0508, or $5.08 per 100 miles.
- Annual cost at 12,000 miles. 12,000 × $0.0508 = $609.52.
Now do the same session on a DC fast charger at $0.48 per kWh. The same 50 kWh billed becomes $24.00, the cost per mile becomes $24.00 ÷ 157.5 = $0.1524, and a full year of that driving would cost 12,000 × $0.1524 = $1,828.57. Fast charging every mile costs $1,219.05 a year more than charging at home in this example — which is the number that decides whether a home charger installation pays for itself.
For comparison against a petrol car, run the same annual mileage through the fuel economy (MPG) calculator: at 30 MPG and $3.45 a gallon, 12,000 miles costs $1,380 in fuel, so home charging is roughly $770 a year cheaper and public fast charging is roughly $450 a year more expensive.
How to read the result
Cost per 100 miles is the number to compare, not cost per kWh. A rate means nothing without an efficiency attached to it. A car doing 4.5 mi/kWh on $0.20 electricity costs less per mile than a car doing 2.5 mi/kWh on $0.14. The per-100-miles figure folds both together and can be compared directly against a gasoline vehicle's fuel cost over the same distance.
Check your rate against your actual bill, not the headline supply price. Most residential bills separate a supply or generation charge from delivery, distribution and taxes, and the number people quote is usually the supply charge alone. The all-in rate is total dollars on the bill divided by total kilowatt-hours, and it is frequently 40-60% higher than the supply component. Using the supply rate alone is the single most common reason a home charging estimate comes in low.
Look at fees as a share of the session, not as a dollar amount. A $1 session fee on a 50 kWh fast charge is 4% of the energy cost. The same fee on a 6 kWh top-up is a third of it. Small top-ups on fee-bearing networks are expensive energy, and that changes how you should plan stops on a long trip.
Do not read the annual figure as your whole cost of ownership. Electricity is one line. Depreciation is usually the largest single cost of running any vehicle, electric or not — put your purchase price and expected holding period through the vehicle depreciation calculator to see the scale of it against a few hundred dollars of electricity.
Finally, remember that the time a session takes is a cost too, even where it is not billed. Work it out with the EV charging time calculator before deciding that a cheaper, slower charger is the better deal.
Cost per 100 miles by rate and vehicle efficiency
| Rate | 2.5 mi/kWh | 3.0 mi/kWh | 3.5 mi/kWh | 4.0 mi/kWh | 4.5 mi/kWh |
|---|---|---|---|---|---|
| $0.10 /kWh | $4.44 | $3.70 | $3.17 | $2.78 | $2.47 |
| $0.16 /kWh | $7.11 | $5.93 | $5.08 | $4.44 | $3.95 |
| $0.25 /kWh | $11.11 | $9.26 | $7.94 | $6.94 | $6.17 |
| $0.35 /kWh | $15.56 | $12.96 | $11.11 | $9.72 | $8.64 |
| $0.48 /kWh | $21.33 | $17.78 | $15.24 | $13.33 | $11.85 |
| $0.60 /kWh | $26.67 | $22.22 | $19.05 | $16.67 | $14.81 |
The $0.16 / 3.5 mi/kWh cell is $5.08, matching the worked example above. Efficiency and rate trade off against each other exactly: doubling one has the same effect as halving the other, which is why a rate alone never tells you what driving costs.
What this calculation leaves out
- Demand charges and fixed monthly fees. Commercial and some residential tariffs bill for peak power draw as well as energy. A fleet depot charging several vehicles at once can find the demand charge exceeds the energy charge.
- Subscription plans. Several charging networks sell a monthly membership that reduces the per-kWh price. Amortise the membership across your expected sessions and enter the effective rate.
- Battery preconditioning and cabin conditioning while plugged in. Both draw from the same meter but never appear as state of charge, so real winter energy per mile can exceed what this model implies.
- Vampire drain. A parked car uses a small amount of energy continuously. Over a month of standing it is measurable; over a session it is not.
- Degradation. Usable capacity falls slowly over the life of the pack, so the kilowatt-hours needed for a given state-of-charge change shrink while the miles they deliver shrink faster.
- Solar self-consumption. If you charge from your own generation, the right rate to enter is your export tariff — the money you gave up by not selling that energy — rather than zero or your import rate.
Home, workplace and public charging compared
Three charging contexts produce three very different costs per mile, and the gap between them is larger than the gap between any two petrol stations. Home charging on a residential tariff is almost always the cheapest, and cheaper still on a time-of-use plan with an overnight window. Workplace charging is often free or heavily subsidised, which makes it the lowest cost of all when it is available. Public DC fast charging is the most expensive, typically several times the residential rate, because you are paying for hardware utilisation, grid connection capacity and convenience as much as for electrons.
That spread is why the useful question is not "what does an EV cost to charge" but "what mix do you actually charge on". A driver who does 90% of charging at home and 10% on the road pays close to the home rate. A driver in an apartment with no home charging pays close to the public rate, and the annual difference between those two situations, at 12,000 miles a year, is comfortably four figures.
Rate structures reward planning. Time-of-use tariffs price overnight energy well below daytime energy specifically to shift load off the peak, and an EV that charges on a schedule is the ideal candidate. Enter your off-peak rate as the main rate and your peak rate as the comparison rate to see what the schedule is worth in a year.
For trip planning, the cost of the electricity is usually smaller than the cost of the time. A stop that adds 150 miles of range at a fast charger might cost $20 and 25 minutes. Work the time side out with the EV charging time calculator, and if you are comparing the trip against taking a petrol vehicle, price that side with the road trip fuel cost calculator.