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.

Charging speed: volts times amps decides the hours

The cost formula ignores how fast you charge, but your schedule does not. Power in kilowatts is volts times amps divided by 1,000, and hours at the plug is energy drawn divided by that power. A Level 1 cord on a standard 120 volt outlet usually pulls about 12 amps, which is 120 x 12 = 1,440 watts, or 1.44 kW. A Level 2 station on a 240 volt circuit protected by a 40 amp breaker can deliver 32 amps continuously, because continuous loads are normally held to 80 percent of the breaker rating: 240 x 32 = 7,680 watts, or 7.68 kW.

Apply that to the 58.33 kWh drawn in the worked example. At 1.44 kW the session takes 58.33 / 1.44 = 40.5 hours. At 7.68 kW it takes 58.33 / 7.68 = 7.6 hours. Those are the two ends of a typical home setup.

Daily driving is the better test. A 40 mile commute in a car that uses 28 kWh per 100 miles at the battery needs 40 x 0.28 = 11.2 kWh added, or 11.2 / 0.90 = 12.44 kWh drawn. Level 1 covers that in 12.44 / 1.44 = 8.6 hours, so an evening-to-morning plug-in is enough. Level 2 does it in 1.6 hours. Speed changes convenience and your ability to hit a cheap rate window, not the price per kWh itself.

The car's onboard charger and the station's breaker each set a ceiling, so use the lower of the two when you estimate hours.

Public charging: when the loss factor double counts

The loss step in the guide matters most when you pay through a utility meter at home. At a public DC fast charger the usual billing basis is different. Many networks charge for the energy the station dispenses to the vehicle, so the conversion loss inside the station is already folded into the posted price and the kWh you are billed for is close to what reaches the battery. If you then divide by 0.90 again, you inflate the estimate.

Take a 75 kWh pack charged from 20 to 80 percent. That is 75 x 0.60 = 45 kWh. At a posted 0.45 dollars per kWh billed on dispensed energy, the session is 45 x 0.45 = 20.25 dollars. In the EV Charging Cost Calculator, set the charging loss field to 0 for this case and you get exactly that figure. Applying a 10 percent loss anyway would give 45 / 0.9 x 0.45 = 22.50 dollars, an overstatement of 2.25 dollars. Practice varies, so read a receipt to see which quantity the network bills.

Some stations bill by the minute instead. Suppose the rate is 0.30 dollars per minute and your car adds those 45 kWh in 30 minutes: you paid 9.00 dollars, or 0.20 dollars per kWh. A slower-charging car that needs 45 minutes for the same energy pays 13.50 dollars, which is 0.30 dollars per kWh. The effective rate is price per minute x minutes / kWh added, so the same sticker price can mean very different costs depending on your car's charge curve and the station's output that day.

A yearly budget that mixes home and public charging

Most owners charge in more than one place, so a single flat rate misleads. Build the budget from a weighted average. Annual energy first: 12,000 miles at 28 kWh per 100 miles at the battery and 10 percent loss is 12,000 x 0.28 / 0.90 = 3,733 kWh billed.

Now split it. If 85 percent of that energy comes from home at 0.15 dollars and 15 percent from public stations at 0.45 dollars, home is 3,173 kWh x 0.15 = 476 dollars and public is 560 kWh x 0.45 = 252 dollars. The year costs 728 dollars, a blended 0.195 dollars per kWh, or about 6.1 cents a mile. Against the 1,400 dollar gas figure from the earlier example, you save 672 dollars.

Shift the mix to 70 percent home and 30 percent public, as happens for apartment dwellers or frequent road-trippers. Home is 2,613 kWh x 0.15 = 392 dollars and public is 1,120 kWh x 0.45 = 504 dollars, for 896 dollars total, a blended 0.24 dollars per kWh and 7.5 cents a mile. Doubling the public share added 168 dollars a year, and the savings against the gas car fell from 672 to 504 dollars.

To run the blended rate through the Gas vs EV Cost Calculator, enter 0.195 as the electricity price and 3.21 miles per kWh as the efficiency; the result lands within a dollar of the 728 dollar hand figure.

Tiered and time-of-use plans: find your real marginal rate

A utility bill often reports an average price, total dollars divided by total kWh, but a car adds energy at the margin. Under a tiered plan the extra kWh land in the most expensive tier. Suppose a household uses 900 kWh a month on a hypothetical plan with 0.13 dollars for the first 600 kWh and 0.22 dollars above that. The bill is 600 x 0.13 + 300 x 0.22 = 144 dollars, an average of 0.16 dollars per kWh.

The car in the earlier example needs about 311 kWh a month. Those units all sit above the 600 kWh line, so the increase is 311 x 0.22 = 68.42 dollars. Using the 0.16 average would have predicted 311 x 0.16 = 49.76 dollars, an 18.66 dollar monthly miss, or roughly 224 dollars a year. Always price the car at the rate of the tier it pushes you into.

Time-of-use plans reward timing. At a hypothetical 0.09 dollars off-peak and 0.28 dollars on-peak, charging 90 percent of the 311 kWh overnight costs 0.9 x 311 x 0.09 + 0.1 x 311 x 0.28 = 25.19 + 8.71 = 33.90 dollars a month. Doing everything at the peak rate costs 311 x 0.28 = 87.08 dollars. Before switching plans, test the whole household, because the rest of your usage may be pricier at peak.

Sanity checks: MPGe and cents per mile

EPA labels use miles per gallon of gasoline equivalent, built on the figure that one gallon of gasoline holds the same energy as 33.7 kWh. That lets you convert your own numbers. The example car draws 31.11 kWh per 100 miles from the wall, so it covers a mile on 0.3111 kWh and rates 33.7 / 0.3111 = 108 MPGe on a wall basis. At the battery, 28 kWh per 100 miles is 33.7 / 0.28 = 120 MPGe.

Cents per mile is just the price per kWh times kWh per mile, so for this car each 10 cents of electricity price is 3.1 cents a mile. At 0.15 dollars that is 4.7 cents, at 0.30 it is 9.3 cents, and at 0.45 it is 14.0 cents, which is already above the 11.7 cents the 30 MPG car spends at 3.50 dollars a gallon. A habit of fast charging at premium rates can cost more per mile than gasoline.

Use ranges to catch typos. Efficient sedans tend to sit in the high 20s of kWh per 100 miles, while large SUVs and trucks can pass 40. A full 0 to 100 percent fill of a 75 kWh pack at 0.15 dollars per kWh with a 10 percent loss is 12.50 dollars, so a home result far above that for one charge points to a typo.

• Price per kWh x 0.311 kWh per mile = cost per mile for the example car
• Cost per mile x 12,000 = yearly cost at 12,000 miles
• Energy drawn = energy added / (1 - loss), except where the network bills dispensed energy