EV Charging Cost Calculator

The formula

Energy into the battery = distance × efficiency (in kWh per km). Energy drawn from the charger = energy into the battery ÷ (1 − loss ÷ 100). Cost = energy drawn × price per kWh. Cost per km = cost ÷ distance.

Efficiency can be entered as kWh per 100 km, kWh per 100 miles, miles per kWh or km per kWh; the calculator converts it to kWh per km first (1 mile = 1.609344 km). If you already know how many kWh went into the battery, choose that start point instead and distance is not needed.

Worked example

A 200 km trip in a car that uses 16 kWh per 100 km needs 200 × 0.16 = 32.00 kWh in the battery. With a 10% charging loss the charger has to supply 32 ÷ 0.9 = 35.56 kWh. At 0.30 per kWh the cost is 35.56 × 0.30 = 10.67, which is 0.053 per km or 0.086 per mile.

What the charging loss means

Some of the electricity drawn from the wall or charger is lost as heat in the charger and battery and never reaches the battery. The loss percentage here is the share of the energy you pay for that is lost, so with 10% loss, 90% of what you draw reaches the battery. The figure varies by car, charger and temperature, so use your own measurement if you have one, such as the difference between the home meter reading and the car's reported energy added. Enter 0 to ignore losses.

Using a rate that is not a single price

• Enter the price per kWh for the charge you are costing. For a public charger billed by time or by session, divide the bill by the kWh delivered to get an effective price per kWh.
• If you charge partly at home and partly at a public charger, work out each separately and add them.
• Time-of-use tariffs change the price during the day, so use the price in force while you charge.

Limits

• The car's efficiency is not constant. Cold weather, high speed, heating and hills raise it, and the calculator uses the one figure you enter.
• It uses a single fixed loss percentage for the whole charge, whereas real losses vary with charging power and battery state.
• It does not include connection fees, idle fees, standing charges, taxes or battery wear.
• Changing the start point or a units box does not convert the numbers already typed in.

More worked examples

From kWh added: 30 kWh into the battery at a 10% loss needs 33.33 kWh from the charger, and at 0.30 per kWh the charge costs 10.00. With 60 kWh added, a 15% loss and 0.45 per kWh, the charger supplies 70.59 kWh and the cost is 31.76, against 27.00 if you ignore losses.

Using miles: 150 miles at 3.5 miles per kWh needs 42.86 kWh in the battery and 47.62 kWh from the charger. At 0.15 per kWh the cost is 7.14, which is 0.030 per km or 0.048 per mile.

Using km per kWh: 500 km at 6.5 km per kWh and no loss is 76.92 kWh. At a price of 8 per kWh the cost is 615.38, which is 1.231 per km.

The four efficiency units

They are just different ways to write the same figure. For a 200 km trip with a 10% loss and 0.30 per kWh, 16 kWh per 100 km, 6.25 km per kWh and 25.75 kWh per 100 miles all give 32.00 kWh in the battery, 35.56 kWh from the charger and a cost of 10.67. 28 kWh per 100 miles is a lower-efficiency car: 34.80 kWh, 38.66 kWh from the charger and a cost of 11.60.

Edge cases

• A charging loss of 100 or more shows "Charging loss must be at least 0% and less than 100%." A blank loss counts as 0: the 200 km example then draws 32.00 kWh from the charger and costs 9.60.
• A price of 0 is accepted, for free charging, and every cost shows 0.00. A blank price shows "Please fill in the electricity price."
• Cost per km and per mile appear only when you start from a distance.