Sustainable Energy Without the Hot Air – Cars

In our everyday lives, between turning on the heater, watching TV or working on the computer, we are often faced with how personal behaviour contributes to climate change. Let’s take a frank look at the environmental impact of cars, without the hot air. The following is an edited extract from Sustainable Energy Without the Hot Air by David MacKay.


For our first chapter on consumption, let’s study that icon of modern civilization: the car with a lone person in it. How much power does a regular car-user consume? Once we know the conversion rates, it’s simple arithmetic:

For the distance travelled per day, let’s use 50 km (30 miles). For the distance per unit of fuel, also known as the economy of the car, let’s use 33 miles per UK gallon (taken from an advertisement for a family car):

(The symbol “≃” means “is approximately equal to.”)

What about the energy per unit of fuel (also called the calorific value or energy density)? Instead of looking it up, it’s fun to estimate this sort of quantity by a bit of lateral thinking. Automobile fuels (whether diesel or petrol) are all hydrocarbons; and hydrocarbons can also be found on our breakfast table, with the calorific value of bread conveniently written on the side: roughly 8 kWh per kg.

Want to know the energy in car fuel? Look at the label on a pack of butter or margarine. The calorific value is 3000 kJ per 100 g, or about 8 kWh per kg. the car in miles per unit volume of fuel, we need to express the calorific value as an energy per unit volume. To turn our fuel’s “8 kWh per kg” (an energy per unit mass) into an energy per unit volume, we need to know the density of the fuel. What’s the density of butter? Well, butter just floats on water, as do fuel-spills, so its density must be a little less than water’s, which is 1 kg per litre. If we guess a density of 0.8 kg per litre, we obtain a calorific value of:

Rather than willfully perpetuate an inaccurate estimate, let’s switch to the actual value, for petrol, of 10 kWh per litre.

Congratulations! We’ve made our first estimate of consumption!

This is the estimate for a typical car-driver driving a typical car today. Later chapters will discuss the average consumption of all the people in Britain, taking into account the fact that not everyone drives. We’ll also discuss in Part II what the consumption could be, with the help of other technologies such as electric cars.

Why does the car deliver 33 miles per gallon? Where’s that energy going? Could we manufacture cars that do 3300 miles per gallon? If we are interested in trying to reduce cars’ consumption, we need to understand the physics behind cars’ consumption. These questions are answered in the accompanying technical chapter, which provides a cartoon theory of cars’ consumption. I encourage you to read the technical chapters if formulae like 1/2mv² don’t give you medical problems.

This chapter’s conclusion: a typical car-driver uses about 40 kWh per day.


If you want to get a closer look at how cars’ fuel consumption compares to other means of transportation, or indeed to renewable energy, then you can find all that info in Sustainable Energy Without the Hot Air available where all good books are sold. Check out our previous blog post on insulation and energy saving.

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