This month I am looking at the way we measure fuel efficiency in our car. I am old enough to remember “Miles per Gallon”. Then we went metric and someone turned it upside-down “Litres per 100km”. Now with EVs, we have kW and kW-hours. So how do they work?

kW (kiloWatts) is a measure of power, which is how fast you are using up energy. For example, our MG4 has motors that can use up energy at the rate of up to 125kW.

kW-hrs (kiloWatt hours) is a measure of total amount of energy stored. For example, our MG4 has a battery that can store 50kW-hrs of energy.

To explain this in another way, let’s build a hypothetical hydro-pumped storage system (and since it is hypothetical, we’ll make it perfectly efficient!) We will put a water tank at the top of Bulli Pass and run a pipe down to near sea level. Then we connect a turbine to the pipe at the bottom of the hill and power it with a 1kw motor. We turn on the motor and pump water up the hill into the tank, and we leave the motor going for 1 hour. The amount of energy used to pump the water up the hill is 1kW x 1 hour, or 1 x 1 = 1kW-hour. So our water tank on the hill now is storing 1kWhr of energy that we can use later.

Now our turbine is reversible and when the water is going back down through the turbine, it can generate at the rate of 1kW. If we let the water run back through the turbine for an hour, the generator will produce a total of 1kW-hour of electrical energy, and by that time our little water tank will be empty. Now if we change and put a 4kW turbine and generator on the end of the pipe, our little water tank would be emptied in ¼ hour. The total energy produced would still be the same. 4kW x 0.25hrs = 1kWhr.

Back to our EV. 50kWhr is the battery’s capacity – that is how much “fuel it has in the tank” or how much energy you have stored. The car has a 125kW motor, which is the maximum rate at which you can use up the power. You can see the problem here: if you suck power out of the 50kWhr battery at 125kW … It will be flat in about 20 minutes! But here’s the thing; even the most committed rev-head does not use 125kW often. Typical average consumption rate around town would be 6 to 10kW which is much more consistent with our experience of driving for many hours before needing a recharge.