Napa's electric bill just got a lot smaller.

Two acres of solar collectors at Lake Hennessey are now generating much of the power needed to pump water from the city's major reservoir.

The project cost $2.6 million, but a grant from PG&E covered nearly half the expense, said Pat Costello, a city water department spokesman.

Over the next 25 years, this solar field will cut the city's electricity bill by more than $3 million. Besides producing a savings for city water customers, the project will benefit the environment, Costello said.

It's as if 1,200 cars were taken off the highway, said Costello. The plant will cut carbon dioxide emissions by an estimated 5,800 tons over 30 years. Solar energy is clean compared to a conventional power plant, he said.

The city contracted with PowerLight Corp., a Bay Area company that also installed a much larger solar-generating facility at Napa Valley College earlier this year.

Hennessey's solar field is at the base of Conn Dam in an area formerly used occasionally for public recreation, Costello said. The gleaming panels are partially visible to motorists through trees along the shoulder of Highway 128.

Pumps at Lake Hennessey move up to 20 million gallons a day up 200 feet to a treatment plant before the water is piped south to the city. These pumps were the city's number one power consumer, costing over $200,000 a year, Costello said.

The new solar installation, which produces 315 kilowatts of power, should meet about half the pumps' electrical demand. Nearly 2,000 photovoltaic panels track the sun to increase their efficiency.

As the cost of regular electricity rises, the long-term fuel bill savings to the city could be more than the $3 million estimated today, Costello said.

The city financed its $1.5 million share of the project with a loan from the Umpqua Bank. The rate of return on this investment should be above 20 percent, to the benefit of water customers, Costello said.

In trying to lower its utility bill, the city explored placing solar panels at other water facilities, Costello said. The city even looked at putting panels atop the downtown parking garages.

The only site that proved cost-effective was at Lake Hennessey, Costello said. PG&E uses revenues from its regular customers to subsidizes the cost of new solar installations.