Thermal numbers

Conventionally generated electricity ranges between 5 and 18 cents per kilowatt hour (the amount of money to get a kilowatt of power for an hour) but in most places it's below 10 cents, according to the Energy Information Agency. Solar thermal costs around 15 to 17 cents a kilowatt hour, according to statistics from Schott, a German company that makes solar thermal equipment.

A solar thermal plant would need a facility to store the heat harvested in the day by its sunlight-concentrating mirrors so that the heat could be used to generate electricity at night.

The plant, ideally, should be capable of generating about 300 megawatts of electricity. Those plants can churn out electricity at about 13 cents a kilowatt.

That's still a relatively high price, so utilities would need to group two, three or more 300-megawatt plants together to share operational resources. They could share control rooms or spare parts. That would knock the price closer to 11 cents a kilowatt hour.

The plants need to be around 500 megawatts in size. Most solar thermal plants right now aren't that big. The 22-year-old thermal plant in California's Mojave Desert is 354 megawatts. Utility company Southern California Edison is erecting a 500-megawatt plant scheduled to open in 2009.

By 2014, solar thermal plants located in the Southwest could crank out nearly 3 gigawatts of power, estimated Travis Bradford of the Prometheus Institute for Sustainable Development, a nonprofit based in Cambridge, Mass. That's enough for about 1 million homes.

Costs can then be reduced further by building the plants close to consumers. It costs about $1.5 million per mile for transmission lines, according to statistics from Acciona Solar Power, which owns solar thermal plants. Solar thermal plants work best in arid deserts that get little rainfall. Since some of the fastest-growing cities in the world are located in sun belts, that's less of a problem than it used to be.

But getting to that point isn't easy. Land-use hearings and permits can drag on for years while construction costs rise. The amount of land required can be an issue too: the 354-megawatt plant in California occupies 1,000 acres. Larger plants would need more land, while smaller plants result in higher costs per kilowatt hour.

Even if all of these factors could be completely optimized, solar thermal power plants would likely not produce electricity at a level that would compete with coal plants. Coal plants, however, will likely be hit with carbon taxes in the near future, which will make solar thermal more competitive. Still, at less than 10 cents a kilowatt, solar thermal would be competitive with electricity from gas-powered plants.