#### forms of energy such as the chemical energy that is fed into a petrol- powered car and the electricity from a wind turbine?

By comparing consumed energy with conceivable produced energy, I do
not wish to imply that all forms of energy are equivalent and interchange-
able. The electrical energy produced by a wind turbine is of no use to
a petrol engine; and petrol is no use if you want to power a television.
In principle, energy can be converted from one form to another, though
conversion entails losses. Fossil-fuel power stations, for example, guzzle
chemical energy and produce electricity (with an efﬁciency of 40% or so).
And aluminium plants guzzle electrical energy to create a product with
high chemical energy – aluminium (with an efﬁciency of 30% or so).

In some summaries of energy production and consumption, all the dif-
ferent forms of energy are put into the same units, but multipliers are
introduced, rating electrical energy from hydroelectricity for example as
being worth 2.5 times more than the chemical energy in oil. This bumping
up of electricity’s effective energy value can be justiﬁed by saying, “well,
1 kWh of electricity is equivalent to 2.5 kWh of oil, because if we put that
much oil into a standard power station it would deliver 40% of 2.5 kWh,
which is 1 kWh of electricity.” In this book, however, I will usually use a
one-to-one conversion rate when comparing different forms of energy. It
is not the case that 2.5 kWh of oil is inescapably equivalent to 1 kWh of
electricity; that just happens to be the perceived exchange rate in a world-
view where oil is used to make electricity. Yes, conversion of chemical
energy to electrical energy is done with this particular inefﬁcient exchange
rate. But electrical energy can also be converted to chemical energy. In an
alternative world (perhaps not far-off) with relatively plentiful electricity
and little oil, we might use electricity to make liquid fuels; in that world
we would surely not use the same exchange rate – each kWh of gasoline
would then cost us something like 3 kWh of electricity! I think the timeless
and scientiﬁc way to summarize and compare energies is to hold 1 kWh
of chemical energy equivalent to 1 kWh of electricity. My choice to use
this one-to-one conversion rate means that some of my sums will look a
bit different from other people’s. (For example, BP’s Statistical Review of
World Energy
rates 1 kWh of electricity as equivalent to 100/38 2.6 kWh
of oil; on the other hand, the government’s Digest of UK Energy Statistics
uses the same one-to-one conversion rate as me.) And I emphasize again,
this choice does not imply that I’m suggesting you could convert either
form of energy directly into the other. Converting chemical energy into
electrical energy always wastes energy, and so does converting electrical
into chemical energy.

## Physics and equations

Throughout the book, my aim is not only to work out numbers indicating
our current energy consumption and conceivable sustainable production, 