Marshall Brain's Blog

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Let's say that you have your typical coal-fired power plant that you are using to produce electricity. How much coal does it take to produce one kilowatt-hour?

If you look at Coal to Electricity, you find this: "Well designed, modern power plants can make one killowatt-hour (1,000 watt hours of electricity) using 9,500 btu's of energy from coal."

How much coal does it take to produce 9,500 BTUs? You can look at this chart to find out: BTUs in different types of coal. What you can see is that the BTU content depends on the type of coal, but roughly a pound of coal can create one kilowatt-hour of electricity.

That seems weird though, because there is something in the back of my head that says it takes a lot less BTUs than that to create a kilowatt-hour. Sure enough, this article confirms that: What is Energy? It says: "One kilowatt-hour equals 3,413 British thermal units and is equivalent to the energy needed to run ten 100-watt light bulbs for one hour." Does it mean that a modern coal-fired powerplant is really that inefficient? 3,413 / 9,500 = 36% efficiency. In other words, nearly two-thirds of the energy in a pound of coal is wasted?

This article shows that it is true: Completion of High-efficiency Coal-fired Power Plant. It points out that the highest-efficiency power plants waste more than half of coal's energy. It says: "Our planned design objective for the No. 4 system was to achieve 44.2% efficiency, the highest level generating efficiency of any coal-fired power plant. By applying the various performance enhancing technologies described in this paper, Hitachi was trying to reach a thermal efficiency of 49.83% in its turbine plant commissions, the highest level ever achieved for this kind of power plant."

On this page you find that gasoline contains 125,000 BTUs per gallon. So you should be able to generate 13 kilowatt-hours from a gasoline-powered generator, if it matched the 36% efficiency of a coal-fired power plant. But gasoline-powered home generators are only (at best) half as efficient as a coal-fired power plant, so you might burn a gallon of gas an hour to obtain 5 kilowatt-hours of electricity. A good long-life diesel generator like this one or this one might generate 10-12 kilowatt hours from a gallon of diesel fuel, which is surprising. You would expect that a huge, fully-optimized power plant would be a lot more efficient than a little diesel generator.

According to this page, the United States is consuming something close to 4 trillion kilowatt-hours of electricity each year.