Published: Monday, April 8, 2013 at 3:00 a.m.

Ten companies and one nonprofit agency are competing for an initial contract to provide electricity to Sonoma County homes and businesses through the county’s proposed public power agency.

The bids are for a contract of at least three years with an estimated worth of about $340 million, according to county officials. The program seeks to displace PG&E as the area’s dominant electricity supplier, a move that supporters say would boost support for renewable energy and spur investment in local power projects.

The responses, received Friday, came mostly from large national and multi-national energy suppliers, including the operator of The Geysers geothermal field along the Sonoma-Lake county border.

A bid by Shell Energy North America could draw scrutiny from activists who have opposed its contract with Marin County’s power agency and its proposed deal with San Francisco.

County officials were not prepared Monday to discuss the power rates they got back in the responses. That cost is seen as the key element determining whether county supervisors launch the program later this year, and whether it will generate support from customers.

Cordel Stillman, the county Water Agency official spearheading the proposal, said he was “thrilled” with the level and quality of responses. He described the list of bidders as “highly competitive.”

“These are real players in the industry,” he said.

The county plans to evaluate the bids over the next week. A report to the Board of Supervisors tentatively scheduled for April 23 could provide the first public look at the range of rates, electricity sources and other terms included in the proposals.

However, a more detailed public look at the actual bids from power suppliers, may never come, at least not at this stage.

The county is withholding release of the documents, citing a 2006 California Supreme Court ruling that shields proprietary information from public disclosure until completion of contract negotiations, often just days before formal approval.

The Press Democrat late Monday submitted a public records request to the county seeking access to the proposals but did not hear back before deadline.

Among the companies that submitted bids was Calpine Corp., the Houston-based operator of The Geysers, already the source of about a quarter of California’s renewable energy. The company is launched on a $700 million, 100-megawatt expansion.

Last year, Calpine officials said the first of two new plants could start production in 2014 if they could land contracts for the energy.

The county is seeking up to 355 megawatts, enough for about 220,000 metered residential and commercial customers, or about 80 percent of those served by PG&E in Sonoma County.

The utility’s power generation costs account for about $180 million a year in residential and commercial billings. : The county program would rollout over three years, with an aim to capture $50 million in 2014, $120 million in 2015, and $170 million in 2016.

It would start with a basic renewable portfolio — including geothermal, wind, solar, small hydroelectric and biomass — of 33 percent. A separate, voluntary portfolio could offer customers a choice of 100 percent renewable power.

Supporters have pushed for an aggressive rollout of local renewable projects to generate energy for the program. On those grounds and others, they’ve opposed a partnership with the Shell Energy North America, a subsidiary of the Dutch fossil fuel giant.

The outlier in the field of bidders was Plumas Rural Services, a nonprofit organization in northeastern California that is looking to sell power from a local wood biomass plant it is buying. The 20-megawatt plant in Loyalton — currently owned by Sierra Pacific Industries — has been idle since 2010 and would support about 14 jobs, said Michele Piller, executive director of the group.

The other energy supply responses came from:

Iberdrola, a multi-national electric utility company based in Spain, with operations throughout New England and New York State.

Noble Americas Energy Solutions, a large U.S. energy retailer based in San Diego.

NRG Energy, based in Houston and Princeton, New Jersey, one of the country’s largest power producers and retailers.

Greensparc Energy Advisors, a San Ramon- and Sacramento-based power development and management firm.

Direct Energy, an energy retailer based in Canada and the U.S., a subsidiary of the British multinational Centrica.

Promet Energy Partners, a natural gas and electricity supplier, based in the Midwest.

ConEdison Solutions, based in Valhalla, New York, a subsidiary of Consolidated Edison.

Constellation, a power and natural gas supplier and subsidiary of Exelon, the Chicago energy producer, trader and distributor.

Billing, metering and transmission would remain with PG&E under any new public venture, and individual customers would be allowed several chances to opt out of the county power agency.

The Board of Supervisors, which now has sole control of the public power agency, the Sonoma Clean Power Authority, is likely to push ahead with the program in two weeks and direct staff to narrow the field of contenders. The same day, it is set to authorize a $250,000 contract with the Berkeley-based consulting firm MIG for marketing and media services.

Cities have yet to commit to the effort and their consent is seen as crucial. Healdsburg is not in the mix because it has its own municipal utility.

To date, the county Water Agency has spent or authorized roughly $700,000 in staff time, studies and consultants on the power proposal.

You can reach Staff Writer Brett Wilkison at 521-5295 or