energy was used in one hour. You could use a gigawatt-hour of energy by
switching on one million toasters for one hour, or by switching on 1000
toasters for 1000 hours.

As I said, I’ll usually quote powers in kWh/d per person. One reason
for liking these personal units is that it makes it much easier to move from
talking about the UK to talking about other countries or regions. For ex-
ample, imagine we are discussing waste incineration and we learn that
UK waste incineration delivers a power of 7 TWh per year and that Den-
mark’s waste incineration delivers 10 TWh per year. Does this help us say
whether Denmark incinerates “more” waste than the UK? While the total
power produced from waste in each country may be interesting, I think
that what we usually want to know is the waste incineration per person.
(For the record, that is: Denmark, 5 kWh/d per person; UK, 0.3 kWh/d
per person. So Danes incinerate about 13 times as much waste as Brits.) To
save ink, I’ll sometimes abbreviate “per person” to “/p”. By discussing ev-
erything per-person from the outset, we end up with a more transportable
book, one that will hopefully be useful for sustainable energy discussions
worldwide.

## Picky details

#### Isn’t energy conserved? We talk about “using” energy, but doesn’t one of the laws of nature say that energy can’t be created or destroyed?

Yes, I’m being imprecise. This is really a book about entropy – a trickier
thing to explain. When we “use up” one kilojoule of energy, what we’re
really doing is taking one kilojoule of energy in a form that has low entropy
(for example, electricity), and converting it into an exactly equal amount
of energy in another form, usually one that has much higher entropy (for
example, hot air or hot water). When we’ve “used” the energy, it’s still
there; but we normally can’t “use” the energy over and over again, because
only low entropy energy is “useful” to us. Sometimes these different grades
of energy are distinguished by adding a label to the units: one kWh(e) is
one kilowatt-hour of electrical energy – the highest grade of energy. One
kWh(th) is one kilowatt-hour of thermal energy – for example the energy
in ten litres of boiling-hot water. Energy lurking in higher-temperature
things is more useful (lower entropy) than energy in tepid things. A third
like electricity.

It’s a convenient but sloppy shorthand to talk about the energy rather
than the entropy, and that is what we’ll do most of the time in this book.
Occasionally, we’ll have to smarten up this sloppiness; for example, when
we discuss refrigeration, power stations, heat pumps, or geothermal power.

#### Are you comparing apples and oranges? Is it valid to compare different

1TWh (one terawatt-hour) is equal to one billion kWh.
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