but also to make clear what these numbers depend on. Understanding what
the numbers depend on is essential if we are to choose sensible policies
to change any of the numbers. Only if we understand the physics behind
energy consumption and energy production can we assess assertions such
as “cars waste 99% of the energy they consume; we could redesign cars so
that they use 100 times less energy.” Is this assertion true? To explain the
answer, I will need to use equations like

However, I recognize that to many readers, such formulae are a foreign lan-
guage. So, here’s my promise: I’ll keep all this foreign-language stuff in techni-
cal chapters at the end of the book. Any reader with a high-school/secondary-
school qualiﬁcation in maths, physics, or chemistry should enjoy these
technical chapters. The main thread of the book (from page 2 to page 250)
is intended to be accessible to everyone who can add, multiply, and divide.
It is especially aimed at our dear elected and unelected representatives, the
Members of Parliament.

One last point, before we get rolling: I don’t know everything about
energy. I don’t have all the answers, and the numbers I offer are open to
revision and correction. (Indeed I expect corrections and will publish them
on the book’s website.) The one thing I am sure of is that the answers to
our sustainable energy questions will involve numbers ; any sane discussion
of sustainable energy requires numbers. This book’s got ’em, and it shows
how to handle them. I hope you enjoy it!

page no.

25The “per second” is already built in to the deﬁnition of the kilowatt. Other examples of units that, like the watt, already
have a “per time” built in are the knot – “our yacht’s speed was ten knots!” (a knot is one nautical mile per hour); the
hertz – “I could hear a buzzing at 50 hertz” (one hertz is a frequency of one cycle per second); the ampere – “the fuse
blows when the current is higher than 13 amps” ( not 13 amps per second); and the horsepower – “that stinking engine
delivers 50 horsepower” ( not 50 horsepower per second, nor 50 horsepower per hour, nor 50 horsepower per day, just
50 horsepower).

Please, never, ever say “one kilowatt per second.” There are speciﬁc, rare exceptions to this rule. If talking about a
growth in demand for power, we might say “British demand is growing at one gigawatt per year.” In Chapter 26 when
I discuss ﬂuctuations in wind power, I will say “one morning, the power delivered by Irish windmills fell at a rate of
84 MW per hour.” Please take care! Just one accidental syllable can lead to confusion: for example, your electricity
meter’s reading is in kilowatt-hours (kWh), not ‘kilowatts-per-hour’.

I’ve provided a chart on p368 to help you translate between kWh per day per person and the other major units in which
powers are discussed.