For our first chapter on consumption, let’s study that icon of modern civi-

lization: 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):

33 miles per imperial gallon ≈ 12 km per litre.

(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 conveniently written on the side:

roughly 8 kWh per kg (figure 3.2). Since we’ve estimated the economy of

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:

8 kWh per kg × 0.8 kg per litre ≈ 7 kWh per litre.

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. I’ve dis-

played this estimate in the left-hand stack in figure 3.3. The red box’s

height represents 40 kWh per day per person.