The most promising aspect of a major contract announced by Sonoma Clean Power on Tuesday concerns what was revealed four days earlier.

The locally controlled electricity provider announced on Friday that it was in the process of hammering out a 10-year deal with the operator of The Geysers geothermal field.

Although the contract calls for the company, a subsidiary of Calpine Corp., to provide just 15 percent of the agency's overall power need, Calpine would be responsible for nearly half of the agency's renewable energy portfolio.

Setting up a deal with Calpine makes sense given Sonoma Clean Power's long-term objective of providing green, locally generated power. The Geysers is about as green and local as it gets. And it's an underutilized source for the local community.

The Geysers, the world's largest geothermal energy field, currently supplies about 25 percent of California's renewable energy. But less than 1 percent of its power is now purchased for Sonoma County customers.

This new long-term contract, which could be approved by the Sonoma County Board of Supervisors today, also encourages the development of renewable energy supplies.

As Staff Writer Brett Wilkison reported on Friday, Calpine has received approval for a $700 million expansion that would add two plants to the 15 in operation at The Geysers. But it hasn't moved forward with the project because it hasn't been able to locate buyers for the new power. This contract would help in that direction.

As Supervisor Shirlee Zane noted, the county wants the company "to expand as we expand and create those local jobs."

At the same time, this contract represents a mere 15 percent of Sonoma Clean Power's energy needs. Thus the more significant news came Tuesday with Sonoma Clean Power's announcement that it had signed a primary energy contract with Chicago-based Constellation, a subsidiary of energy giant Exelon.

Sonoma Clean Power officials say the three-year contract will allow them to be competitive with rates by Pacific Gas and Electric Co., but, at this point, few specifics have been released. Geof Syphers, interim CEO of Sonoma Clean Power, said he was need to withhold the price of the contract and other details because his agency was still in negotiation with Calpine for the secondary supply contract.

These deals come with a good deal of promise — and promises — but, so far, they also arrive with limited detail, particularly concerning the costs to consumers. The public has cause for optimism, but it would be wise to reserve judgment until that fine print is available.