EV Charging Cost Calculator

How the cost of a charge is worked out

A home charge costs the energy that leaves your wall socket times your price per kWh. The tricky part is that this is more than the energy that ends up in the battery. The charger and the battery turn some of it into heat, so the calculator works in two steps.

1. Energy into the battery = battery size × (target − start). A 75 kWh battery taken from 20% to 80% gains 45 kWh.
2. Energy from the wall = battery energy ÷ charging efficiency. At 88% that is 51.14 kWh.

The cost is the second figure times your price. The price box starts at the US residential average from the EIA, with its period printed beside it. Replace it with the rate on your own bill.

Many quick estimates multiply the battery size by the price and stop there. That assumes no losses and a charge from empty to full. Entering your real start and target, and counting the losses, gives a figure that matches the bill.

Efficiency: Level 1 and Level 2

Level 2 charging at 240 V is the more efficient of the two. Studies put it at about 88 to 90%, and the default here is 88%. One US study measured about 84% on Level 1, a 120 V outlet, because the same fixed losses are spread over a smaller flow of energy for much longer. That is a single study, and the general 12 to 15% loss figure only bounds it, so treat 84% as the least firm number on this page and type your own if you have measured it. The level you pick moves the efficiency and the breaker to that level’s starting values. If you type your own, the level button leaves your number alone.

Charger amps, kW and time

A charger is limited to 80% of its breaker. That is the same rule seen from the breaker’s side: a continuous load, and EV charging counts as one, is sized at 125% of its current. The relevant sections are NEC 625.41 and 625.42, and they are cited here by number only. Your code edition and your local inspector decide what applies at your house.

In practice that gives these pairs, which Tesla and ChargePoint print in their own manuals: a 20 A breaker runs a 16 A charger, a 30 A breaker 24 A, a 40 A breaker 32 A, a 50 A breaker 40 A and a 60 A breaker 48 A. Power is volts times amps. At 240 V, 40 A is 9.60 kW. Commercial Level 2 sites often run at 208 V, so the level button has a 208 V choice.

Charge time is the energy from the wall divided by the charger’s kW, shown up to the next 5 minutes so it is never shorter than the math. If your car cannot take the full charger power, type its AC limit in the extra options and the smaller number is used. Charging can slow near full, so treat the time as a plan.

The table under the calculator lists each setting with its breaker, the time to add 45 kWh and the miles gained per hour of charging. Every cell is computed from the same formulas.

Miles added and cost per mile

The car’s label figure, kWh per 100 miles, already includes charging losses, because it is measured from the wall. So the miles added are the energy from the wall divided by that figure. The calculator does not apply the efficiency a second time. You can find your car’s number on the window sticker or on fueleconomy.gov. Recent models run from 23.1 to 71.7, and most fall between 29 and 45.5.

Cost per mile is the kWh per 100 miles times your price, divided by 100. The gas comparison divides the gas price by your gas car’s mpg. The tool also shows the electricity price where the two match, so you can see how much room you have.

What amps do I need for my daily miles?

Open “Amps for my daily miles”, type how far you drive each day and how many hours the car is plugged in. The tool works out the power needed, the amps at your voltage, the smallest charger setting that covers it from 16, 24, 32, 40 and 48 A, and the breaker that setting needs. Above 48 A it says you need more than one charger or a longer charging window.

What this calculator leaves out

It does not model public fast charging, time-of-use plans, tax credits, battery health or the exact charging curve of a particular car. It also does not size the wire for your charger. That depends on the length of the run and the code edition, and the wire size calculator is the place to start.

Worked example

1. Inputs: 75 kWh battery from 20% to 80%, 240 V on a 50 A breaker, 88% efficient, 18.19 ¢ per kWh.
2. Into the battery = capacity × (target − start) = 75 kWh × (80% − 20%) = 45 kWh
3. From the wall = battery ÷ efficiency = 45 kWh ÷ 0.88 = 51.14 kWh
4. Cost = from the wall × price = 51.14 kWh × 18.19 ¢ = $9.30
5. Charger = volts × amps = 240 V × 40 A = 9.60 kW
6. Time = from the wall ÷ charger kW = 51.14 kWh ÷ 9.60 kW = 5.33 h
7. Miles = from the wall ÷ (kWh per 100 miles ÷ 100) = 51.14 kWh ÷ 0.3 = 170 mi

Questions people ask