How big is big? Try 25,000 solar panels spread out over 12 football fields big. That’s Recurrent Energy’s Sunset Reservoir solar project big.

Crews are now busy laying photovoltaic panels atop one of the city’s largest water reservoirs, the seismically-safe northern side of the Sunset Reservoir, which takes up eight whole city blocks between 24th and 28th avenues and Quintara and Ortega streets.

When completed, the public-private enterprise will have built the biggest municipal solar farm in California and possibly the United States, according to city officials, who on Tuesday toured the installation, still under construction.

The panels will generate five megawatts of energy for the city, which uses its own power — not PG&E’s — to run Muni vehicles and light lights in select municipal buildings like City Hall. That comes at a cost of about $2 million a year over 25 years, according to the power-purchase agreement which allowed the project to go forward. San Francisco will be able to buy the project outright from Recurrent for roughly $30 million in seven years, should city coffers be flush by then.

Some 8,000 panels have already been installed on the project, which was slated to be completed by now, according to a May 2009 proclamation from the company. Construction did not begin until March 2010, and should be completed by this September. They expect to flip the “on” switch in December, according to Public Utilities spokesman Tyrone Jue.

That the project has even progressed this far was great news for officials from both City Hall and the Public Utilities Commission, many of whom expressed amazement that any of this even happened (privately, of course. The pen was down, guys, come on!).

Of course, any green project means green jobs, and this one’s got ’em: 53 green-collar jobs, according to Mayor Gavin Newsom, who helped one of those jobs install a solar panel Tuesday (but who did not earn a union card, not yet).

Not invited to — and not even informed of, as he tells it — the Tuesday fete was Supervisor Ross Mirkarimi, an outspoken clean energy advocate who was until recently a member of the Green Party. Mirkarimi, some recall, was vocal in his opposition to the power-purchase agreement part of the project, suggesting the city could have pushed for a better deal.

He said Tuesday that he’ll adopt a wait-and-see approach before casting any further judgment on the project. “If it delivers,” all will be well, he said.

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