The other day on 6PR Kent Roberts said that on his way into work he turned off his rear window demister, to save, he said, a bit of fuel.

He made the point that he realized that it was probably inconsequential, but he did it anyway.

So let’s do a ballpark calculation to work out how much it was.

I have no idea where Kent lives, or what car he drives, but for the sake of the calculation, let’s assume he lives 50km away and drives a car that gets fuel consumption of 10L/100km.

If we assume that his rear window demister draws 5 Amps, and it’s a 12V system, this means that this requires 12 x 5 = 60W (Watts, the unit of power).

Let’s now assume that his car, while he is driving it, is generating 60kW of power. This is 60,000W.

Now, if the rear window demister is using up 60W of power, this is 1/1000th of the power that the engine is producing. So the engine must work harder, by a factor of 1/1000 to operate the rear window demister.

So this is also the same factor by which the fuel consumption increases.

If he is driving 50km, and his car uses 10L per every 100km, this means that he has used 5L of fuel.

Now, since his engine has to work harder by a factor of 1/1000, this means that his engine will use an extra 5L/1000 = 5mL

So by using his rear window demister, he has used an extra 5mL of fuel. Or to put it another way, by not using it, he has saved 5mL.

Which equates to about 0.75c, at current fuel prices.

So if he did it for a year, he would save himself about enough for a packet of chips.

I reckon that’s worth it Kent, don’t you?