This week a letter in our local paper gave me pause for thought. The author stated that if all the cars in Switzerland (3.8 million of them) drove for an hour a day every day for a year with their headlights on they will consume 35’000 tonnes more crude oil than if the lights were turned off. Can this be correct?
The average power of a car headlight bulb is about 70 Watts (http://hypertextbook.com/facts/2005/YatManTsui.shtml)
So one car’s headlights consume 140 W of power
Per hour this gives 140 * 60 * 60 =500 KJ of energy
So per year one car’s headlights will consume 365 * 500 = 182 MJ of energy
One litre of petrol contains 35 MJ (http://en.wikipedia.org/wiki/Gasoline)
So assuming 100% efficiency 182 MJ corresponds to 5.2 l of petrol. So one car driving around for one hour each day for a year will consume 5.2 l more fuel because of the lights. Actually, the efficiency of converting the chemical energy in the petrol to electrical energy is more like 25% so probably it extra fuel is more like 20 l).
The density of petrol is 0.74 kg/l (http://en.wikipedia.org/wiki/Gasoline#Density) so 5.2 l is about 3.8 kg
Now crude oil contains 21 to 35% petrol, for simplicity call it 30% (http://wiki.answers.com/Q/How_much_gasoline_can_be_made_from_one_barrel_of_crude_oil).
This gives 3.8/0.3 = 12.7 kg of crude oil. So for the 3.8 million cars in Switzerland this gives 48’300 tonnes of crude oil. Considering all the approximations my figure and that of the letter in the newspaper are pretty close: so, I would say that ‘yes’ the correspondent is probably correct.
However what I find interesting is how my perception of the figures is altered by the scale; using up an extra 5.2 l of fuel a year by leaving the lights on does not seem that bad, especially when offset by the increase in road safety, but when taken over the whole country the increase of 48’000 tonnes of crude oil is shocking, irresponsible even. The trouble is I have a good idea of what 5.2 l of petrol looks like, but no idea abound 48’000 tonnes of crude, it sounds like a hell of a lot!
So let’s look at that 5.2 l in another way, how far I drive on it? In 2012 the EU will require new cars to have an efficiency of 5 l per 100 km, so I can go about 100 km on it. If I’m driving at a steady 60 km/hr for one hour a day 365 days a year I’ll cover about 22’000 km. This consumes 4’200 l of petrol or taking the calculation above 39’000’000 tonnes of crude oil Switzerland wide per year (4’200 * 0.74 * 3.8 * 1’000’000 /0.3). Suppose everyone left the car at home one day a week, this would reduce the consumption by about 14% (1 – (365-52)/365) or 5’500’000 tonnes of crude a year. So maybe a better headline would be:
‘Car share one day a week and save 5’500’000 tonnes of crude oil a year!’
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