How many cows does it take to power a light bulb?

LAS VEGAS--Well, the specifics will depend on the light bulb, the amount of manure produced by the cow, and the processes employed to convert the manure into a more easily consumable form of energy like methane.

But a cow is probably good for about 100 watts, according to Ryan Katofsky, an associate director at Navigant Consulting, during a session at the Power-Gen conference taking place this week in Las Vegas. "You can probably get up to 300 to 400 watts, depending how rapidly you can digest it," he said.

Thus, a single cow can run a light bulb, and in some cases Bessie can crank up the lights for a good part of the house. ("Digest" refers to the process the manure undergoes in a piece of farm equipment called a digester. Digesters essentially accelerate the natural decay process by heating the manure and attacking it with microorganisms.)

Biomass, i.e. garbage, cow manure and rotting farm products, can't displace coal or natural gas, but it can put a dent in fossil fuel consumption. If you harvested all the manure from the cows in the country, you could probably generate about 300 to 500 megawatts of power, Katofsky guesstimated. One of the companies trying to popularize cow power is Microgy.

Landfills can also be tapped for methane. In the U.S. there are about 360 ongoing projects for converting landfill matter into energy. In all, they produce about 1,200 megawatts, which is about the same amount of energy produced as a large coal-fired power plant. Another 52 projects are under construction and there are about 1,900 sites in the U.S. that hold some potential for being tapped. Some producers consume the methane as methane, and others exploit the harvested gases to generate electricity.

In fact, until a few years ago, biomass produced more energy than wind and solar combined. Pulp and paper conglomerates use their leftovers to run their own facilities.