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