Energy density

] Energy density
Main article: Energy value of coal
The energy density of coal, i.e. its heating value, is roughly 24 megajoules per kilogram.[38]
The energy density of coal can also be expressed in kilowatt-hours for some unit of mass, the units that electricity is most commonly sold in, to estimate how much coal is required to power electrical appliances. One kilowatt-hour is 3.6 MJ, so the energy density of coal is 6.67 kW·h/kg. The typical thermodynamic efficiency of coal power plants is about 30%, so of the 6.67 kW·h of energy per kilogram of coal, 30% of that—2.0 kW·h/kg—can successfully be turned into electricity; the rest is waste heat. So coal power plants obtain approximately 2.0 kW·h per kilogram of burned coal.
As an example, running one 100 watt lightbulb for one year requires 876 kW·h (100 W × 24 h/day × 365 {days in a year} = 876000 W·h = 876 kW·h). Converting this power usage into physical coal consumption:

It takes 438 kg (966 lb) of coal to power a computer for one full year.[39] One should also take into account transmission and distribution losses caused by resistance and heating in the power lines, which is in the order of 5–10%, depending on distance from the power station and other factors.