HERE COMES THE SUN: South San Joaquin Irrigation District board member Dale Kuil checks the wiring on one of the panels at the district's new solar farm in Oakdale. The farm, which cost around $7.8 million to build, will save the district money in energy costs.

Sunrays falling on a 10-acre plot of solar panels in Oakdale have quietly begun to produce electricity for the South San Joaquin Irrigation District’s water treatment plant next door.

Since the district’s solar farm was first switched on to the electricity grid on May 15, the captured sunlight has provided enough electricity to cover about two-thirds of the plant’s power needs. The plant cleans water from the Stanislaus River, which is then piped west into local city water systems in Manteca, Lathrop and Tracy.

The irrigation district paid the Pacific Gas and Electric Co. more than $439,000 last year to provide electricity for the plant. With the completion of the $7.8 million solar project, most of those costs should be eliminated, district manager Jeff Shields said.

The district also expects to get about $5.2 million from the state over the next five years through the California Solar Initiative rebate program.

The idea to build the solar farm began to take shape about two years ago, as growing electricity bills for the water treatment plant got the attention of the district’s board.

Shields considered several solutions, including a long-distance connection to a cheaper power provider, the Modesto Irrigation District, but he eventually settled on the idea of a solar farm.

The district hired the German-based company SunTechnics to produce 6,720 Photovoltaic panels — each 4 feet long and 6 feet wide — to be set up on district land next to the water treatment plant.

In September, workers began to erect the one-megawatt solar array, which is built on an automatically rotating axis so that its panels face the sun directly throughout the day.

The amount of energy it produces makes this the largest solar farm in Northern California, Shields said.

Because the solar farm produces slightly more power than the treatment plant uses during the day, excess electricity is fed back onto the local power grid.

This electricity that goes out during the daytime is credited against the plant’s power usage during the night, when the solar farm captures no energy.

Since the solar farm was switched on, it has been generating slightly more energy than SunTechnics engineers predicted, Shields said, but there are still problems with some of the meters that keep track of the total output.

The district has remained quiet about the project while engineers work through the final kinks, but there are plans for a opening ceremony later this month that will likely feature congressman Jerry McNerney (D-Pleasanton), a former wind-energy consultant.