In a previous instalment of this column, I wrote about Mercedes-Benz going to produce an electric car.

By this I mean the automaker is going to produce a car for sale to the public that will be 100% electrical in drive – it will not be a hybrid car with a generator that switches on when needed to support the electric motor and battery system. The car (now available in the US) is a B-model Mercedes-Benz and has a lithium-ion battery and a 132 kW motor. It sells for $40 000.

In an Internet post, the company says the car does 85 miles to the gallon (electric). This is abbreviated as MPGe. I quote from Wikipedia: “ The MPGe metric was introduced in November 2010 by the EPA (Environmental Protection Agency) . . . the ratings are based on the EPA’s formula, in which 33.7 kWh (121 megajoules) of electricity is equivalent to one gallon of [petrol].”

So, we can assume that the vehicle does 85 miles/137 km for every 33.7 kWh consumed. Let us say you’re driving around town, at an average speed of 60 km/h. After 2.3 hours, you have used 33.7 kWh.

So, we can more or less say that on the average commute of 2.5 hours you are going to go home having used 33.7 kWh. And now you plug the car in. If we assume that the lithium-ion battery charges up in 60 minutes, you are going to have to take about 100 kWh from your house supply (more than the 33.7 kW you used since the battery is not very efficient).

This is never going to work since the supply to the house is not more than 13 kW, so you have to change the charging time to about seven hours. This is doable, if somewhat limiting – if you want to pop down to the pub, you will have to use the bakkie. However, while the power supply to the house is capable of supplying 13 kW, a group of houses fed from one mini substation has an average consumption (called the after-diversity demand) of about 4 kW for each house. Thus, if several houses are each trying to draw 13 kW to charge electric cars, then the supply at the miniature substation to the group of houses will trip.

Getting back to that 132 kW motor . . . really that size? To use 33.7 kWh in 2.5 hours, the motor will only be running at an average of one-tenth of the rated power. Thus, you cruise at 60 km/h on not much power. What this means is that, when you pull up at the traffic lights and the home boy racer next to you gives it the beans, you can take off in a cloud of molten rubber, silently, leaving your man dazed and confused.

As one would expect from Mercedes-Benz, it seems that the company worked all the numbers. What is not worked out or Mercedes-Benz does not seem to be able to work out is the seven-hour charge restriction as a result of the limitations of the municipal power supply to a domestic residence.

But then again, if electric cars get to be very popular, perhaps they will be charged while people are at work at special charging stations. This sounds desirable but really is not going to pan out because there is going to be even more congestion in the cities than at present. What is possible is for there to be battery-change stations where you pull up and they take the battery out and give you another one. This does not seem very likely, as it will require all electric manufacturers to have the same sort of battery, which, if we go by cellphones, is not going to happen.

But, quite probably, it will be just simple – get home, plug the car in, crack open a beer and watch television. Perhaps some sort of local area system will communicate with all electric cars charging such that the miniature substation is not overloaded.

The whole area is a topic in which I have much interest. I drove an Alan Cocconi electric car in 1993. I have been waiting for a long time for this.