A back of the envelope guide to the physics of electric vehicles

Is Australia on the way to an electric vehicle future? Some people would like to think so, but is it feasible? It is time to scribble on the back of an envelope about watts.

According to the Australian Bureau of Statistics there are currently (as at January 2021) 19.8 million registered vehicles in Australia, including passenger cars, vans, trucks and motorcycles. Cars and light commercial vans make up the vast bulk of the registrations. If we assume that if those vehicles were electric, they would have a 75kwh battery that would require a full charge once per week, then the extra demand for electricity across the country for a full year would be 74,250 Mwh.

Total electricity generation in Australia during 2019 was 265Twh, according to energy.gov.au. Of that, 19% was produced by renewable sources of solar, wind, hydro and biomass. So, the generation supplied by renewables is around 50,000Mwh per year. The generation of power from renewable sources would have to increase from its current level by over 150% to cope with the demands from EV charging.

Photo by Narcisa Aciko on Pexels.com

The alert reader will have noticed that it would take only a small increase in the power generated by non-renewable sources (ie coal, gas and oil) to drive a national EV fleet, but what would be the point of that? It is much more efficient to power a car with petrol than it is to burn coal to generate power and store that in a battery. So if we are on the way to an EV fleet, it must be powered by renewable sources. Anyone driving an EV now is most likely much more damaging to the environment than are drivers of petrol cars. But that inconvenient truth does not inhibit smugness. It was Winston Churchill who once said of a political opponent, “Occasionally he stumbled over the truth, but hastily picked himself up and hurried on as if nothing had happened.” It is a phrase that often comes to mind when listening to fans of Teslas.

Another interesting figure comes out of the additional 74,250 Mwh requirement. Charging batteries takes a lot of time. At the typical current rate of 3kw charging at home, it would take around 25 hours to charge a 75kwh battery. Fast charging rates rise to around 7kw, so 10 hours to fully charge. High speed chargers are available at around 50kw hence theoretically 1.5 hours to charge. But, high speed charging is very damaging to a battery’s health, so it should be done rarely – maybe a top-up out on the road. Clearly, most charging will have to be done overnight at home. This means solar power generation is not an option. Windmills will be needed. As will wind. One of those big windmill turbines dotted around the country can generate 2 Mw at theoretical maximum capacity. But in practice, efficiency losses reduce that to less than 50% and then wind speeds either too high or too slow cut that down to zero. A realistic expectation of average generation over a year out of 1 turbine is 0.5 Mw but the power is actually delivered at either 0 or 1 Mw, depending on conditions. In total, the country would need a further 150,000 large wind turbines to power an EV fleet. The country would also need the wind to blow at the right speed in the right places at the right time, but that is another matter.

Physics is not in favour of an EV fleet. However, the politics of the issue are potentially damaging. Do not underestimate the ability of governments to mess things up by rules, regulations, taxes and subsidies.