How much does it cost to charge an electric car?

The three-step formula

Electric cars are fueled in kilowatt-hours (kWh), and the cost of a charge is simply how many kWh you buy times the price of each one. There are three steps, and the middle one is what most quick estimates skip.

Step one, energy added: battery capacity x the share of the battery you are filling. Step two, energy drawn: divide the energy added by (1 - loss), because not everything pulled from the wall makes it into the battery. Step three, cost: energy drawn x price per kWh.

• Energy added = capacity x (charge-to % - charge-from %) / 100
• Energy drawn = energy added / (1 - loss %)
• Cost = energy drawn x price per kWh

A worked example: 75 kWh battery, 20 to 90 percent

Take a 75 kWh battery charged from 20 percent to 90 percent at home, with electricity at 0.15 dollars per kWh and 10 percent charging loss.

Energy added = 75 x (90 - 20) / 100 = 75 x 0.70 = 52.5 kWh. Energy drawn = 52.5 / (1 - 0.10) = 52.5 / 0.90 = 58.33 kWh. Cost = 58.33 x 0.15 = 8.75 dollars.

If you had skipped the loss step you would have estimated 52.5 x 0.15 = 7.88 dollars, a 87-cent error on one charge. Over a year of weekly charges that is about 45 dollars of cost you did not plan for.

Why losses are divided, not subtracted

Charging loses some energy as heat, mostly in converting AC from the wall to the DC the battery stores. FuelEconomy.gov, drawing on published studies and Idaho National Laboratory data, says battery charging efficiency varies but is often 84 to 93 percent. Flip that around and the loss is roughly 7 to 16 percent of what you draw.

Because the loss is a share of the energy drawn, not of the energy delivered, you divide by (1 - loss) rather than adding the percentage. A 10 percent loss means dividing by 0.90, which adds 11.1 percent to the energy billed. A 15 percent loss adds 17.6 percent, and 25 percent adds a full 33.3 percent. The gap widens as the loss grows.

On the 75 kWh example at 0.15 dollars per kWh, the effect looks like this: 0 percent loss costs 7.88 dollars; 7 percent costs 8.47 dollars (56.45 kWh drawn); 10 percent costs 8.75 dollars (58.33 kWh); 15 percent costs 9.26 dollars (61.76 kWh); and 16 percent costs 9.38 dollars (62.50 kWh). The spread across the realistic range is about 90 cents per charge, a smaller lever than price per kWh but not zero.

You can measure your own loss. Compare what the wall meter or charger recorded to what the car says it added. If the meter shows 58.3 kWh and the car reports 52.5 kWh added, then 1 - 52.5 / 58.3 = 10 percent.

Price per kWh: the lever that matters most

The price you pay per kWh swings far more than the loss does. The Alternative Fuels Data Center notes that actual charging costs depend on the equipment, the time of day, and how long the station is used, and that some utilities offer residential time-of-use rates and other incentives. Many drivers can cover daily needs on Level 1 charging overnight, which requires no extra installation.

Put some prices on the same 58.33 kWh drawn. These rates are illustrative, so replace them with the numbers from your own bill and charging apps. At 0.10 dollars per kWh (an overnight time-of-use rate) the charge costs 5.83 dollars. At 0.15 dollars it costs 8.75. At 0.30 dollars (a peak rate) it costs 17.50. At 0.45 dollars (a hypothetical public fast-charging price) it costs 26.25, three times the home rate for exactly the same energy.

The AFDC page works through its own example at 10.7 cents per kWh: a 54 kWh battery fully depleted costs about 6 dollars to fill, and a vehicle that uses 27 kWh per 100 miles costs about 3 cents per mile. Checking the math, 54 x 0.107 = 5.78 dollars before any losses, and 6.42 dollars once a 10 percent loss is applied. 27 x 0.107 = 2.89 dollars per 100 miles. That price is an old example, so use your own rate, but it confirms the method.

From cost per charge to cost per mile and per year

Per-charge cost is useful, but cost per mile is how you compare with a gas car. You need your EV's efficiency. Suppose the car uses 28 kWh per 100 miles measured at the battery, a hypothetical figure for this example.

Include the loss: 28 / 0.90 = 31.11 kWh drawn per 100 miles. At 0.15 dollars per kWh that is 4.67 dollars per 100 miles, or about 4.7 cents a mile. Over 12,000 miles that is 120 x 31.11 = 3,733 kWh and 560 dollars.

Compare a 30 MPG gas car with fuel at 3.50 dollars a gallon: 12,000 / 30 x 3.50 = 1,400 dollars a year. The difference is 840 dollars of fuel and energy. Flip the question and ask what electricity price would erase that gap. A gas car costs 3.50 / 30 = 11.67 cents per mile; the EV uses 0.311 kWh per mile drawn, so break-even is 0.1167 / 0.3111 = 0.375 dollars per kWh. At prices far below that, the EV wins on energy cost, and the advantage disappears if you charge at premium public rates all the time.

To run this in the Gas vs EV Cost Calculator, enter efficiency in miles per kWh. From the wall, 100 / 31.11 = 3.21 miles per kWh. Using the wall figure keeps the loss inside the result.

When the estimate breaks down

The formula is a good planning tool, but several real-world factors move the answer, and they are worth knowing before you trust a number to the penny.

• Cold weather: FuelEconomy.gov reports that EV fuel economy can drop roughly 39 percent in mixed driving in cold conditions (20 degrees Fahrenheit versus 77), with about two-thirds of the extra energy heating the cabin. On the 28 kWh per 100 miles car, that makes cost per 100 miles about 4.67 / 0.61 = 7.65 dollars.
• Blank charge fields: if you leave the starting and target charge empty, the calculator assumes a full 0 to 100 percent charge. On this example that is 75 kWh added, 83.33 kWh drawn, and 12.50 dollars.
• Billing by time or session: some public networks charge per minute or add session or idle fees instead of pricing purely per kWh, and a per-kWh formula will not capture that.
• Charge-speed taper: fast charging slows at high states of charge. That changes how long you sit there, not how many kWh you buy, so the cost formula holds even when the time does not.
• Usable versus total capacity: a battery's total capacity is usually larger than the usable amount. If you use the total, the cost estimate runs slightly high.
• Tiered or demand-based home rates: some plans charge more per kWh after you pass a monthly threshold, so a single flat rate understates the cost of heavy charging months.