How much coal does it take to power a 100 watt lightbulb for a year?

A calendar, a lightbulb and a pile of coal

It takes 800 pounds of coal to generate the electricity for a 100-watt lightbulb for a year.

In terms of size, 800 pounds of coal would fill a hot tub. But if you take it all the way back to its origins, more than 300 million years earlier (check out our upcoming article on the origins of coal), the original dead plants would need to have been up 10-times that size, or 8,000 pounds (4 tons).  Just to make it fun, assuming the plants back then were anything like what we have today, four tons would be 12 or so 20-foot trees, each with a lifespan of a century.

If you really break it down, your year-long 100-watt lightbulb is actually powered by sunlight – captured by a dozen trees over the course of a century, and then stored for 300 million years. In modern technology terms, coal is really just like a disposable battery, created by mother nature back before there were dinosaurs.

Diving into the math of lightbulbs

For those of you who like to see the math, it’s laid out below.

First, a 100-watt bulb operating for a full year requires 876 kilowatt hours (kWh) of electricity,

     24 hours 
   x 365 days 
   x 100 watts 
   = 876,000 watt hours

   ÷ 1,000 watt hours per kilowatt hours
   = 876 kilowatt hours

Next, a ton of coal contains 20.025 million British thermal units (BTUs) of heat (source: EIA). 

Assuming the average coal plant is 37% efficient (source: Wikipedia), burning a ton of coal leaves 7.41 million BTUs of usable energy.

     20.025 million BTUs per ton of coal
   × 37% coal plant efficiency
   = 7.41 million BTUs of usable electricity from burned coal

Since 1 million BTUs is the same as 293 kilowatt hours (kWh), a ton of coal can generate 2,168 kWh of electricity.

     7.41 million BTUs 
   x 293 kilowatt hours per million BTUs 
   = 2,168 kilowatt hours per ton of burned coal

This means we need 0.40 tons or 800 pounds of coal to power our lightbulb for a year

       876 kWh for a year of light
   ÷ 2,168 kWh per ton of coal
   = 0.40 tons of coal per year of light

   0.40 tons = 800 pounds

Answer: 800 pounds of coal

800 pounds of coal, enough to power a lightbulb, takes up about a hot tub's worth of volume

Additional reading

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