The energy stored in an EV battery is measured in kilowatt hours (kWh). For instance, if a kettle uses typically 1kW of electrical power and was left boiling for 1 hour, it would consume 1kW x 1hr =1kWh of energy. For reference 1kWh of energy is equivalent to 3.6 MegaJoules (MJ) [or 86grams of oil equivalent (note not diesel or petrol)].
The fuel efficiency of the electric vehicle may be compared to a petrol vehicle using the following formula:
1 kWh = 3.6 MegaJoules which would require approximately 0.1125 Litre of Petrol or 0.1 Litre of Diesel to produce (assuming Lower Heating Value for Petrol of 32MJ/L and Diesel of 36MJ/L).
The following example shows how to compare EV fuel efficiency in terms of L/100km and MPG equivalent for Diesel or Petrol
How to Compare Direct Fuel Efficiency
This section can be used to give the Consumer an approximate idea of how to compare the fuel efficiency of the EV compares in terms of L/km or Miles per Gallon (mpg) assuming either a Petrol or Diesel fuel was used to supply the same amount of energy. The fuel efficiency quoted by the manufacturers will be in Wh/km. This indicates the amount of electricity which will be required from the plug socket per km travelled according to the NEDC driving cycle.
Note when comparing the Petrol or Diesel results, the fuel calorific values assumed in the conversion are 32MJ/L (Petrol) and 36MJ/L (Diesel), therefore the only difference in performance is due to the fossil fuel energy densities and the vehicle efficiency (ie Wh/km) is exactly the same.
This provides a measure of Consumption Efficiency because the Consumer is purchasing units of Electricity from the Home/On-Street/Fast Charging point rather than purchasing Petrol or Diesel at a fuel station. As the Electricity in turn must be generated at a power station or wind farm, this does not therefore provide an overall measure of fuel cycle efficiency (or “Indirect” Fuel Efficiency) instead it provides a measure of the efficiency of the EV itself (or “Direct” Fuel Efficiency).
For a discussion on the Well to Wheels or Fuel Cycle Efficiency (Indirect Fuel Efficiency).
To convert Wh/km to L/100km or mpg assuming Petrol Fuel Energy Equivalent
L/100km = (Wh/km) x 0.01125
mpg = 25125 ÷ (Wh/km)
An EV with an efficiency of 200Wh/km will have equivalent efficiency of 2.25L/100km or 126mpg assuming petrol fuel
To convert Wh/km to L/100km or mpg assuming Diesel Fuel Energy Equivalent:
L/100km = (Wh/km) x 0.010
mpg = 28249 ÷ (Wh/km)
An EV with an efficiency of 200Wh/km will have equivalent efficiency of 2.0L/100km or 141mpg assuming diesel fuel