Modern agriculture is the use of land to convert petroleum into food.
We’ve already discussed in Chapter 6 how much sustainable power
could be produced through greenery; in this chapter we discuss how much
power is currently consumed in giving us our daily bread.
A moderately active person with a weight of 65 kg consumes food with
a chemical energy content of about 2600 “Calories” per day. A “Calorie,” in
food circles, is actually 1000 chemist’s calories (1 kcal). 2600 “Calories” per
day is about 3 kWh per day. Most of this energy eventually escapes from
the body as heat, so one function of a typical person is to act as a space
heaterwith an output of a little over 100 W, a medium-power lightbulb. Put
10 people in a small cold room, and you can switch off the 1 kW convection
How much energy do we actually consume in order to get our 3 kWh
per day? If we enlarge our viewpoint to include the inevitable upstream
costs of food production, then we may find that our energy footprint is
substantially bigger. It depends if we are vegan, vegetarian or carnivore.
The vegan has the smallest inevitable footprint: 3 kWh per day of en-
ergy from the plants he eats.
I love milk. If I drinka-pinta-milka-day, what energy does that require? A
typical dairy cow produces 16 litres of milk per day. So my one pint per
day (half a litre per day) requires that I employ 1⁄32 of a cow. Oh, hang on
– I love cheese too. And to make 1 kg of Irish Cheddar takes about 9 kg of
milk. So consuming 50 g of cheese per day requires the production of an
extra 450 g of milk. OK: my milk and cheese habit requires that I employ
1⁄16 of a cow. And how much power does it take to run a cow? Well,
if a cow weighing 450 kg has similar energy requirements per kilogram
to a human (whose 65 kg burns 3 kWh per day) then the cow must be
using about 21 kWh/d. Does this extrapolation from human to cow make
you uneasy? Let’s check these numbers: www.dairyaustralia.com.au says
that a suckling cow of weight 450 kg needs 85 MJ/d, which is 24 kWh/d.
Great, our guess wasn’t far off! So my 1⁄16 share of a cow has an energy
consumption of about 1.5 kWh per day. This figure ignores other energy
costs involved in persuading the cow to make milk and the milk to turn to
cheese, and of getting the milk and cheese to travel from her to me. We’ll
cover some of these costs when we discuss freight and supermarkets in