For nearly three years, Sonoma County has been pursuing a dream of creating its own power agency, one that gives customers the assurance that when they turn on their lights, the source of that electricity is not damaging the planet. At least not to the extent that existing sources, used by Pacific Gas & Electric Co., are.

The process included a survey a year ago that found about 80 percent of county residents favored the idea, along with about three-quarters of businesses. But support dropped significantly when potential customers were asked if they would be willing to pay more — even a little more — for a locally operated system.

The problem is that paying less or the same amount for such a system is not an option. Breaking away from PG&E and tapping into more renewable energy sources would come at a price. The question is how much.

Sonoma County will get its first look at how much today.

In late February, the Sonoma County Water Agency, the lead agency behind this proposed community choice aggregation plan, requested bids from companies interested in running the Sonoma Clean Power system. Today is the deadline for turning in those bids.

One company that's expected to make a pitch is Shell Energy North America, the supplier for Marin Clean Energy, Marin County's community choice aggregation system. But the hope is that Sonoma County will receive more than one bid and that this county will get a better deal — specifically one that commits more to locally produced renewables.

More important, however, is that this is the moment the county will get its first look at the bottom line — how much more residents, businesses and public agencies can expect to pay to be part of this new system. These numbers are critical because the contract is based on the commitment of eight cities in the county and, as yet, none has committed. (Healdsburg has its own municipal utility district and won't be participating.) The contract also anticipates that rates will be attractive enough that only 20 percent of residential customers would opt out and stay with PG&E. To make that work, the cost increases must be modest.

A study two years ago projected that the typical customer would likely pay an average of $4 to $10 more per month over a 20-year period for power provided by the county versus power supplied by PG&amp;E. <NO1>The difference could rise up to $15 more per month in 2017 before dropping to as low as $1 more per month in 2032.

But the difference projected in San Francisco, which is moving ahead with a similar system, is far greater than that. Average customers there are expected to see rates increase around 50 percent for a power supply that would be considered 100 percent renewable. According to San Francisco Public Utility Commission reports, electricity rates could nearly double for some customers. This has caused much debate in San Francisco, where a decision on a not-to-exceed rate for the CleanPowerSF has been put on hold until April 23.

Meanwhile, PG&amp;E is moving ahead with a green energy program that would give customers the option of buying more power from renewable sources, which has the potential to increase the opt-out rates for systems such as the one being contemplated in Sonoma County.

It's a given that reducing greenhouse gas emissions and improving energy efficiency isn't optional. Sonoma County is committed to cutting carbon emissions 25 percent from 1990 levels by 2015. Having a local clean power authority would help achieve that goal while encouraging the production of more local renewable energy sources.

All the same, Sonoma County needs to commit to taking a sober look at what the bids say about what this will cost the county, residential ratepayers and businesses in the long term. Sonoma County can't afford to get this wrong.