Sunlight is free. But turning that solar energy into electricity has been a lot more expensive than getting electricity from fossil fuels like coal. That's beginning to change. And it could make a big difference in our energy choices.

Several solar-power companies are racing to beat the "buck per watt" barrier. If solar companies can bring the cost of generating solar power below a dollar, you'll be able to pay roughly the same for clean electricity as you do now for energy generated using coal.

As it is, power produced from coal plants has cost about a third less per-kilowatt hour than solar power.

One company says it's already below that buck per watt barrier. First Solar says it's cut its manufacturing cost for solar modules to 98 cents per watt. Less than a buck. That would make it the first company to reach that price range.

Congress may also have an effect. It is currently debating whether to put a price on carbon-dioxide emissions. Something solar power doesn't have to worry about.