On AC the car's onboard charger caps the power (often 7–11 kW); check that it can take the charger's full rating.
Adding 45 kWh (20% → 80%) draws 50 kWh from the grid: about 6 h 56 min on a 7.2 kW charger for $8.00, good for about 281 km. Per 100 km the EV costs $2.84 against $12.25 for the petrol car.
Energy added is the battery's usable capacity times the change in charge level, and dividing by the charging efficiency gives the energy drawn from the grid. Charging time is that grid energy divided by the charger's power, and the cost is grid energy times the price per kWh. The car's energy use turns the same figures into range added and a running cost per 100 km or 100 miles, set against a petrol car.
The defaults take a 75 kWh battery from 20% to 80%: 45 kWh into the battery, 50 kWh from the grid at 90% efficiency, just under 7 hours on a 7.2 kW charger (the output of most home Level 2 units, per the US Department of Energy) and 8.00 at 0.16 per kWh. That adds about 281 km at 16 kWh/100 km.
The time assumes constant power. DC fast chargers slow as the battery fills, and on AC the car's onboard charger can cap the rate below the charger's rating.
Divide the energy needed by the charger's power. Taking a 75 kWh battery from 20% to 80% draws 50 kWh from the grid, which is 6.9 hours at 7.2 kW. The US Department of Energy's Alternative Fuels Data Center puts Level 1 (120 V) at about 5 miles of range per hour, Level 2 at about 25 miles per hour, and DC fast charging at 100 to 200+ miles per 30 minutes.
How much does it cost to charge an electric car?
Multiply the energy drawn from the grid, including charging losses, by your price per kWh. The default session puts 45 kWh into the battery, draws 50 kWh at 90% efficiency and costs 8.00 at 0.16 per kWh. At 16 kWh/100 km that is 2.84 per 100 km, against 12.25 for a petrol car using 7 L/100 km at 1.75 a litre.
How much energy is lost when charging an EV?
Typically 7% to 16%. fueleconomy.gov, citing Idaho National Laboratory data, says battery charging efficiency is often 84% to 93%, so a 45 kWh top-up draws roughly 48 to 54 kWh from the meter. The 90% default sits in that range. The EPA names the charging cable and the car's onboard charger as the places where the energy is lost.
Should I enter the EPA rating or my car's own energy figure?
Either, but set the efficiency to match. The EPA measures energy at the wall outlet, including losses in the charging cable and onboard charger, so an EPA kWh/100 mi figure already counts charging losses: set efficiency to 100%. Keep the 90% default for a figure that measures energy taken from the battery.
Does cold weather reduce EV range?
Yes, mostly through cabin heating. fueleconomy.gov, citing AAA's 2019 tests, reports that at 20 °F (−7 °C), compared with 75 °F (24 °C), EV range fell about 41% with the cabin heater in use and about 12% without it. Raise the energy-use figure in winter to see the effect on range and cost.
How accurate is the EV charging cost and time calculator?
Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 3 worked examples whose answers come from independent sources; for example, “75 kWh battery, 20 → 80% on a 7.2 kW charger” is checked against Hand calculation: 75 × 0.6 = 45 kWh ÷ 0.9 = 50 kWh ÷ 7.2 kW = 6.944 h × 0.16 = 8.00; 16 ÷ 0.9 × 0.16 = 2.844 per 100 km; 7 × 1.75 = 12.25.
Where does the method come from?
US DOE Alternative Fuels Data Center — charging electric vehicles at home (levels and power); fueleconomy.gov — electric vehicle energy use and cost comparison; US DOE Alternative Fuels Data Center — charging levels and range added per hour; US EPA — fuel economy and EV range testing (MPGe measured at the wall outlet).