## October 21, 2004

### Energy units

Here is a primer on the units one tends to encounter in researching energy issues.

The watt (W) is a measure of electrical power. (Power is the rate of doing work or producing or expending energy.) One watt is equal to 1 joule (J) per second.

The joule is a measure of energy, or the ability or capacity to do work. Other measures of energy are
• kilowatt-hour (KW-h), a thousand watts of power produced or used for one hour, equivalent to 3.6 MJ.

One PJ = 277.78 TW-h.

When a 1-MW [maximum rate of energy generation] wind turbine produces at 25% of that capacity as averaged over a year, its annual output is

1 MW × 0.25 × 365 days × 24 hours = 2,190 MW-h.

• British thermal unit (Btu), equivalent to 1,054.8 J or 0.293 W-h.

Quadrillion Btu = 1.055 PJ = 293 TW-h.

• million tonne oil equivalent (mtoe), equivalent to 41,868 MJ or 11,630 KW-h.
The metric system prefixes:
K means kilo, a thousand, or 103
M means mega, a million, or 106
G means giga, a billion, or 109
T means tera, a trillion, or 1012
P means peta, a quadrillion, or 1015
E means exa, a thousand times more than peta, or 1018