Volume of coal per kWh

Thread Starter


Joined Aug 22, 2007
I found this article at howstuffworks interesting:

How much coal is required to run a 100-watt light bulb 24 hours a day for a year?
Here is a summary (my words & format) of their results (with a small correction):

Wattage x Hours in a year = Total energy consumed
0.1 kW x 8,760 hours = 876 kWh

Generator efficiency x Thermal energy content of coal = Energy per ton of coal
0.4 x 6,150 kWh = 2,460 kWh/ton

Total energy / Energy per ton of coal = Coal mass
876 kWh / 2,460 kWh/ton = 0.356 tons = 712 lbs = 323 kg
So they end up with an answer of 712 lbs or 323 kg.
I wondered how big a pile of coal that was.

The density of coal is 1.1 - 1.5 g/cm^3 (average 1.3g/cm^3).
323000 g / 1.3 g/cm^3 = 248461 cm^3 (a cube 63cm or 25in on a side)
= 0.25 cubic meters = 8.8 cubic feet = 248 L = 66 gallons (55 imperial)

That's per year. And per day it's

248 L/yr = .68 L/day = .18 gallons = .72 quarts = 2.9 cups = 1.45 pints

Here are the figures per kWh:

.28 L = .075 gallons = .30 quarts = .6 pints = 1.2 cups

So, on the order of a cup (or quarter liter) per kWh.

[BTW, is there a list of useable tags?]


Joined Apr 2, 2007
a minor clarification
by generator efficiency you/they mean plant/overall efficiency i suppose.
generator efficiency are quite high.(210/215 IIRC)


Joined May 16, 2005
What kind of coal? I've seen figures range from 7000 to 12500 BTU per pound for coal. (That would be roughly 4.5 to 8.13 KWH per kilogram.) The density of coal also varies by both type and purity. Also, the size of a pile of coal will vary based on how finely broken up it is.


Joined Apr 2, 2007
that is very true.
generally power plants are designed for the coal that wud be available to them(thus depending on geography of the region).
calorific value as well as volatile content etc varies from various grades of coal bituminous, lignite, peat, etc.
hence blending of coal is done to get the calorific value for which the plant is designed.
however the plant efficiency remains almost unaffected. i:e 0.4 with all regeneration and reheating etc.