Energy Units


The first step in understanding the energy problem is understanding the numbers; the following gives an overview of the major energy units used in various different reports on energy use. If you're not sure what a 'quad' is, or how it compares to a TW or a billion bboe, read on!

The metric unit for energy is the joule (1J).
For power, or energy use per unit time, the unit is the watt (1W).
The metric time unit is the second (1 s) so

1 J = 1 W * 1 s

It also helps to be familiar with the standard metric prefixes for large numbers, since we'll be dealing with some very large numbers here. Each of these is 1000 times larger than the previous:
Mmega-1 million
Ggiga-10^9 (1 billion)
Ttera-10^12 (1 trillion)
Ppeta-10^15 (1 quadrillion)

Many other energy and time units are frequently used in reporting energy industry data:
Unit name equivalent to
1 BtuBritish thermal unit1055 J
1 kWhkilowatt-hour3.6 MJ (megajoule)
1 kWhkilowatt-hour3412 Btu
1 bboebarrel of oil equivalent5.8 million Btu
1 bboebarrel of oil equivalent6.12 GJ (gigajoule)
1 bboebarrel of oil equivalent1700 kWh
1 toe(metric) ton of oil equivalent39.7 million Btu
1 toe(metric) ton of oil equivalent41.9 GJ
1 tce(metric) ton of coal equivalent27.8 million Btu
1 tce(metric) ton of coal equivalent29.3 GJ
1 short ton coal2000 lb of coal equivalent25 million Btu
Note that coal has only about 70% the energy content of oil, for the same weight.
The following units are on the scale of yearly world energy consumption (1 year = 8766 hours or 31.5 million seconds):
1 TWhterawatt-hour1 billion kWh
1 TWyterawatt-year8766 TWh
1 TWyterawatt-year31.5 EJ (exajoule)
1 quadquadrillion (10^15) Btu1.055 EJ
1 quadquadrillion Btu293 TWh
1 TWyterawatt-year29.9 quad
1 quadquadrillion Btu 172 million bboe

Very roughly, 1 quad is about the same as 1 EJ, 1 TW or 1000 1 GW power plants generate 30 quads of energy in a year (but see the next paragraph on the issue of heat rate), and a billion barrels of oil can yield about 6 quads of energy.

An additional important number is the heat to power ratio (or "heat rate") for an electricity production system; the number of kWh (thermal) or Btu of fuel burning required to produce 1 kWhe (electric). For comparison between different types of energy source in electric power production, a common heat rate of about 3 kWh/kWhe, or 10,400 Btu/kWhe, typical of fossil fuel steam generators, is useful. At a heat rate of 3 kWh/kWhe, 1 bboe is just under 600 kWhe, ie. you get about 600 kWh of useful electric energy from burning 1 barrel of oil. See this EIA table for actual numbers from a range of power sources.

See efunda for a more comprehensive list of energy conversions. The Energy Information Administration's conversion chart has some useful details too.

Created: 2005-01-21 15:47:46 by Arthur Smith
Modified: 2005-03-11 22:52:03 by Arthur Smith