economics of sustainable power are: there’s simply not enough sustainable
power to support our current lifestyle; massive change is coming.”

Energy and power

Most discussions of energy consumption and production are confusing
because of the proliferation of units in which energy and power are mea-
sured, from “tons of oil equivalent” to “terawatt-hours” (TWh) and “exa-
joules” (EJ). Nobody but a specialist has a feeling for what “a barrel of oil”
or “a million BTUs” means in human terms. In this book, we’ll express
everything in a single set of personal units that everyone can relate to.

The unit of energy I have chosen is the kilowatt-hour (kWh). This
quantity is called “one unit” on electricity bills, and it costs a domestic user
about 10p in the UK in 2008. As we’ll see, most individual daily choices
involve amounts of energy equal to small numbers of kilowatt-hours.

When we discuss powers (rates at which we use or produce energy),
the main unit will be the kilowatt-hour per day (kWh/d). We’ll also occa-
sionally use the watt (40 W 1kWh/d) and the kilowatt (1 kW = 1000 W
= 24 kWh/d), as I’ll explain below. The kilowatt-hour per day is a nice
human-sized unit: most personal energy-guzzling activities guzzle at a
rate of a small number of kilowatt-hours per day. For example, one 40 W
lightbulb, kept switched on all the time, uses one kilowatt-hour per day.
Some electricity companies include graphs in their electricity bills, show-
ing energy consumption in kilowatt-hours per day. I’ll use the same unit
for all forms of power, not just electricity. Petrol consumption, gas con-
sumption, coal consumption: I’ll measure all these powers in kilowatt-
hours per day. Let me make this clear: for some people, the word “power”
means only electrical energy consumption. But this book concerns all forms
of energy consumption and production, and I will use the word “power”
for all of them.

One kilowatt-hour per day is roughly the power you could get from
one human servant. The number of kilowatt-hours per day you use is thus
the effective number of servants you have working for you.

People use the two terms energy and power interchangeably in ordi-
nary speech, but in this book we must stick rigorously to their scientific
definitions. Power is the rate at which something uses energy.

Maybe a good way to explain energy and power is by an analogy with
water and water-flow from taps. If you want a drink of water, you want a
volume of water – one litre, perhaps (if you’re thirsty). When you turn on a
tap, you create a flow of water – one litre per minute, say, if the tap yields
only a trickle; or 10 litres per minute, from a more generous tap. You can
get the same volume (one litre) either by running the trickling tap for one
minute, or by running the generous tap for one tenth of a minute. The
volume delivered in a particular time is equal to the flow multiplied by the

Figure 2.1. Distinguishing energy and power. Each of these 60W light bulbs has a power of 60W when switched on; it doesn’t have an “energy” of 60W. The bulb uses 60W of electrical power when it’s on; it emits 60 W of power in the form of light and heat (mainly the latter).
is measured in
is measured in
litres per minute
is measured in
is measured in
kWh per day