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

http://science.howstuffworks.com/question481.htm

So they end up with an answer of 712 lbs or 323 kg.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

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?]