Grams equivalent per kilowatt-hour (gCO2eq/kWh) is a unit of measurement of the amount of greenhouse gas, expressed in terms of the equivalent amount of carbon dioxide, that is released for every kilowatt-hour of electricity generated or consumed.
This metric is used to compare the global warming effects of different energy sources, products or activities, each having unique mixtures of greenhouse gas outputs, each type of gas with different global warming potentials. That is, it includes not just carbon dioxide but also other greenhouse gases, the main ones of which are methane, nitrous oxide and fluorinated gases (roughly 27 to 32 individual gases, depending on the classification).
An example of the great differences in greenhouse effects of various gases is the difference between carbon dioxide and methane. Methane is a far more potent greenhouse gas when comparing their warming effects, particularly over shorter time frames. Over a 20-year period, it has more than 80 times the warming power of carbon dioxide because of its much greater heat-trapping ability. (However, this drops to roughly 30 times over a hundred year period because carbon dioxide can remain in the atmosphere far longer than methane.)
For example, typical life-cycle grams of carbon dioxide equivalent per kilowatt-hour for various electric power generation types (including construction, operation and decommissioning) include 820 to 1000 for coal, 400 to 500 for natural gas, 41 to 48 for solar photovoltaic, 12 for nuclear, 11 to 14 for onshore wind, and ten to 40 for hydropower.
Typical grams of carbon dioxide equivalent per passenger kilometer for various modes of inter-city passenger transportation modes are 230 to 250 for air travel, 120 to 180 for internal combustion engine automobile, 70 to 100 for diesel rail, 40 to 60 for bus, and four to eleven for high-speed rail using low carbon output power sources.