The program, which started in February, allows FPL's residential customers to pay $9.75 a month on top of their regular utility bills for 1,000 kilowatt hours of renewable energy.

FPL, the utility arm of Juno Beach-based FPL Group Inc. (NYS: FPL, $64.63), will use tradeable energy credits to buy renewable energy and transport it to customers' homes.

For every 10,000 customers who sign up, FPL has pledged to build 150 kilowatts of solar operations in Florida. The utility is looking at areas in Miami and plans to announce a site in a few weeks, FPL spokeswoman Pat Davis said.

"We're looking forward to hitting that 10,000 mark," Davis said.

FPL's program is one of 121 in the Southeast and 500 across the nation. In terms of customer participation, the program is No. 2 in the Southeast, behind the Tennessee Valley Authority's program, which has 7,200 participants and puts out 72 megawatts of additional wind power.

The Sunshine Energy program is also on track to be among the top 10 green-power programs in the nation, in terms of customer participation, according to the Energy Department's National Renewable Energy Laboratory in Golden, Colo.

Organizations are basing the program's popularity on its low price and on FPL's success in getting the word out. The company is marketing Sunshine Energy through Austin, Texas-based Green Mountain Energy Co., the nation's largest retail provider of green energy.

Andy Prince, a spokesman for Green Mountain Energy, said the company spoke to customer focus groups before the program started, has sent out direct mail and included information in FPL's monthly bills.

It is often the utilities and not state agencies that end up taking the initiative behind green-power programs, said Blair Swezey, a principal policy adviser with the National Renewable Energy Laboratory.

"You can think of these initial customers as being early adopters that are leading the way for a product that eventually will become mainstream," Swezey said.