The project’s advantages are obvious: electricity generated from the sun will allow FPL to cut natural gas use and reduce carbon dioxide emissions. It will provide extra power when it is most needed: when the summer sun is shining, Floridians are cranking up their air-conditioning and electricity demand is at its highest.

The plant also serves as a real-life test on how to reduce the cost of solar power, which remains much more expensive than most other forms of electrical generation. FPL Group, the parent company of Florida Power and Light, expects to cut costs by about 20 percent compared with a stand-alone solar facility, since it does not have to build a new steam turbine or new high-power transmission lines.

“We’d love to tell you that solar power is as economic as fossil fuels, but the reality is that it is not,” Lewis Hay III, FPL’s chairman and chief executive, said on a recent tour of the plant. “We have got to figure out ways to get costs down. As we saw with wind power, a lot has to do with scale.”

For solar power, scale is still a relative term. At its peak, the solar plant will be able to generate 75 megawatts of power, enough for about 11,000 homes. But that is dwarfed by the adjacent gas plant, which can produce about 3,800 megawatts of power. (A megawatt is enough to power a Wal-Mart store.)

Utilities are being pulled in different directions. They must ensure that the lights remain on at all times as well as provide the lowest-cost power to their customers. At the same time, they are being pressed to find ways to reduce their greenhouse gas emissions and invest in renewable power sources.