A tank full of petrol ("gasoline," if you prefer), and what it means

We bought petrol today. It's quite a novelty for us, as we don't do it very often. The last time we bought petrol was way back on August 27th... 2007.

I've mentioned before that we're not really all that enthusiastic about driving, but today we made a short journey in our car and needed to fill up the tank.

It made me think: what exactly have we bought today ? Our tank full of fuel came to a bit more than 36 and a half litres. 36.5 l of petrol weighs about 26 kg. Burning that one tank full of petrol will put about 84 kg, a little more than my own weight, of CO2 into the air. That's the effect from just one tank full of petrol, which according to the manufacturer is supposed to be enough to drive around 500 km.

Petrol is amazing stuff. It has an energy content of 9.7 kWh per litre, so today we bought 355 kWh of energy for our 53 euros. The car consumes about 0.7 kWh produces 0.17 kg of CO2 per km driven.

A few years back, someone who set a world record by cycling over 1000 km in 24 hours calculated that his average power output was about 115 W for the entire 24 hours. That's someone at the absolute peak of what is possible, and his body generated about 2.7 kWh over a 24 hour period.

In light of this, I think it's reasonable to say that an averagely strong person doing an 8 hour shift at a physically demanding job would expend no more than 1 kWh per day at work, so we could say that in our tank full of petrol we've bought the equivalent of someone working hard nearly every day for an entire year, including weekends and almost all holidays. This cost just 53 euros. That really is a bargain.

Car manufacturers put a lot of effort into insinuating otherwise, but cars are actually quite amazingly inefficient. It seems no-one is really all that interested in making them any more efficient. Petrol provides an awesome amount of energy for a remarkably small cost, and pushing gently on the accelerator makes a car zoom along quite nicely. Anyone who's ever pushed a car any distance knows that there is nothing particularly efficient about it.

There is much hype over electric cars, even though they do little more than to shift the source of the output of that CO2 elsewhere. Many power stations are still coal power stations. In fact, many new coal power stations are still being built, all around the world.

When you burn coal at a power station you get only about 2 kWh of energy per kg of coal burnt. If instead of filling up a petrol car today we had charged batteries of an electric car with the same amount of energy by using electricity generated by coal, it would have required the burning of no less than 177 kg of coal, producing a whopping 518 kg of CO2 in the process. That's over 6 times the amount produced by burning petrol directly in a car. An equivalent electric car therefore would still consume about 0.7 kWh but produce over 1 kg of CO2 per km driven. That's being quite generous, as you never get so much energy out of a battery as you put into it when you charge it. With NiMH batteries, for instance, you get back out only about 2/3rds of the energy you put in - under ideal conditions.

Electric bicycles are also not so innocent as many people think. According to this pro electric bike website it costs 1.8 kWh in electricity to charge an electric bike battery for a 20 mile (32 km) range. That's nearly 1 kg of coal per charge. Even with their figures, which like mine for electric cars ignore the problem of inefficiency of rechargeable batteries, this indicates that an electric assisted bicycle, powered by electricity from coal, consumes about 0.05 kWh and produces about 0.08 kg of CO2 per km ridden.

Yes, so electric bikes, if charged by electricity generated by coal (in Australia, for instance, 80% of electricity comes from coal, and the majority of the rest from other fossil fuels), have a CO2 output per km which is only about half that of an average car with one occupant.

Public transport isn't all that much better either. The average bus occupancy in the UK is only about 9 people. Due to this, figures you can calculate for overall efficiency for the entire bus network are actually remarkably similar to those for single occupancy cars.

The same happens with trains. Only if you assume that electric trains take all their electricity from carbon neutral sources do they look particularly good. High speed trains are remarkably close in emissions to jet aircraft.

Here's a comparison of different passenger transport modes in the US. It doesn't much matter than it says BTU instead of kWh that I've been using. 3413 BTU are equivalent to 1 kWh, so divide these figures by 3413 to get equivalent figures to my calculations above:

As you can see, changing from one of these modes to another is really only about making a small change. Anyone who tells you that one mode of powered transport is incredibly more efficient than another is either selling you something, or is misinformed. However, it's big business (much like oil is) and there are plenty of people with a vested interest in trying to convince you that their motorised car replacement can change the world.

Lots of things are described as "green" but few genuinely are. There is a truly "green" alternative to these modes of transport. It's called a bicycle. No fuel is burnt. No CO2 emissions result. You do have to eat, however from what I've seen, people who drive don't eat any less than people who cycle.

All bicycles are wildly more efficient than powered vehicles. They have to be. Inefficiency makes you tired too quickly (compare pushing a bike with pushing a car). However, some bicycles are still more efficient than others and this makes a huge difference if you need to travel more than short distances. A truly "green" car-replacement for longer journeys looks like this !


The sharp eyed might notice we went to a BP station. I know some people are boycotting them, but frankly it's a waste of time doing so. In my opinion there is very little to choose between oil companies. They're all doing their best to make as much profit as possible from extracting oil from wherever they can get it, and they are all responsible for spills. BP was simply unlucky enough to have had their disaster where it was widely seen and reported upon. Boycotting one chain won't do any good. What really needs to happen is for people to appreciate what an amazing resource both oil and coal are, and to consume less of them.