Thing measured unit name symbol value
humans person p
mass ton t 1 t = 1000 kg
gigaton Gt 1 Gt = 109 × 1000 kg = 1 Pg
transport person-kilometre p-km
transport ton-kilometre t-km
volume litre l 1 l = 0.001 m3
area square kilometre sq km, km2 1 sq km = 106 m2
hectare ha 1 ha = 104 m2
Wales 1 Wales = 21 000 km2
London (Greater London) 1 London = 1580 km2
energy Dinorwig 1 Dinorwig = 9 GWh

Billions, millions, and other people’s prefixes

Throughout this book “a billion” (1 bn) means a standard American billion,
that is, 109, or a thousand million. A trillion is 1012. The standard prefix
meaning “billion” (109) is “giga.”

In continental Europe, the abbreviations Mio and Mrd denote a million
and billion respectively. Mrd is short for milliard, which means 109.

The abbreviation m is often used to mean million, but this abbreviation
is incompatible with the SI – think of mg (milligram) for example. So I
don’t use m to mean million. Where some people use m, I replace it by M.
For example, I use Mtoe for million tons of oil equivalent, and MtCO2 for
million tons of CO2.

Annoying units

There’s a whole bunch of commonly used units that are annoying for various
reasons. I’ve figured out what some of them mean. I list them here,
to help you translate the media stories you read.


The “home” is commonly used when describing the power of renewable
facilities. For example, “The £300 million Whitelee wind farm’s 140 turbines
will generate 322 MW – enough to power 200 000 homes.” The
“home” is defined by the BritishWind Energy Association to be a power of
4700 kWh per year []. That’s 0.54 kW,
or 13 kWh per day. (A few other organizations use 4000 kWh/y per household.)

The “home” annoys me because I worry that people confuse it with the
total power consumption of the occupants of a home
– but the latter is actually