Torque Shop

For electric cars, what do battery size in kilowatt-hours (kWh) and charging in kW mean?

For petrol and diesel engines, energy is stored in liquid form in the fuel tank. Liquid fuel is transported and sold by volume measured in litres but electricity, whether "transported" or sold is measured in kWh. Hence, battery storage is also measured in kWh, which is an energy unit.

Your domestic electricity is billed according to how much energy you use. Hence if you switched on five 100-watt bulbs for 10 hours, the energy consumed by these bulbs would be 5kWh (number of bulbs x watts x hours). At the current domestic electricity tariff of 23.65 cents per kWh, this would cost $1.1825.

In an electric vehicle, a 40kWh battery is able to supply a constant 40kW of power for one hour or 20kW for two hours.

In theory, a 3.7kW charger - the current standard home-based charger that is supplied by car manufacturers - would take 10.8 hours to charge the battery from zero to 40kWh.

In practice, however, electric car batteries never deplete to zero charge and rarely do they actually achieve 100 per cent charge. So the charge indicator will likely show full charge after about eight hours.

Logically, a charger with 10 times the power would perform the same charge in one-tenth the time.

In any case, the energy consumption of an electric vehicle is measured in kWh. It ranges between 11 and 13kWh per 100km, or 9.1 and 7.7km/kWh.

Shreejit Changaroth

A version of this article appeared in the print edition of The Straits Times on October 13, 2018, with the headline 'Torque Shop'. Print Edition | Subscribe