The San Mateo County Board of Supervisors unanimously approved an agreement Tuesday that allows Sheriff Greg Munks to transfer some inmates to Alameda County's jail if needed.

The two-year contract, which can be extended, authorizes paying Alameda County up to $3 million.

Munks told supervisors that although San Mateo County's jails are overcrowded, he doesn't anticipate having to execute the agreement he negotiated with Alameda County Sheriff Gregory Ahern.

It would only be used if the number of inmates reaches a tipping point or there's some other reason -- such as flooding at the women's jail -- the extra space is needed before the county's new jail in Redwood City opens in late 2015.

"I do not anticipate having to utilize this, but it's something we want to have," Munks said. "We just want to have it in place in the event that the population spikes between now and completion of the jail project."

The Maguire Correctional Facility for men has an average daily population in the "low 900s," more than the state capacity limit of 688, Munks said.

And the women's jail, with a capacity limit of 84, has an average of about 130 inmates.

"That's pretty overcrowded," Munks said.

Board President Dave Pine asked the sheriff what number would trigger a decision to move some inmates to Alameda County.

Munks said the county could accommodate no more than 1,100 male and 150 female inmates. The number of women inmates surged as a result of the state's "realignment" decision to send some lower-level offenders to county jails instead of state prisons.

"If I had to bet we were going to use any of those beds I would guess it would be with the female population," Munks said. "They're closer to the trigger point in terms of running out of space."

Among the kinds of inmates who would be sent to Alameda County are those with longer sentences, those who don't have many court appearances or medical appointments or those who live in East Bay, Munks said.

Supervisor Don Horsley, the county's former sheriff, asked Munks whether he would consider using more electronic monitoring to reduce the number of inmates.

Munks replied he believes incarceration is the best option for some offenders to give them structure and protect the public. Electronic monitoring works better for inmates who are transitioned out of jail back into the community, he said.

"I'm not a big fan of electronic home detention," Munks said. "Many of the things that led up to them ending up in jail have to do with their environment and the lack of structures that exist at home. ... I don't think there's a large population of people sitting there who could go home and do better."

Jail helps nudge offenders to turn their life around, Munks added.

"In my opinion, sitting at home watching TV doesn't get people ready to change," he said.

Horsley countered he doesn't see "sitting in jail watching television is the greatest option" for inmates either, but he acknowledged he and Munks "fundamentally disagreed" on the matter.

Before voting, Supervisor Adrienne Tissier said it's unfortunate the county may have to outsource inmates, but if conditions at the aging women's jail continue to deteriorate she'd hate to see the county "throwing good money after bad."

It's because of overcrowded jails that the supervisors decided in 2011 to construct a second facility. The new one, estimated to cost $215 million, is being built on a five-acre site east of Highway 101 in Redwood City.

Alameda County also has sealed deals with Sonoma and Monterey counties to house extra inmates at its below-capacity Santa Rita Jail in Dublin.

According to the contract approved Tuesday, San Mateo County could transfer an unspecified number of inmates at a cost of $125 a day each for a group of one to 15 inmates and $85 a day each for a group of 46 or more.

Alameda County used to contract with the state to house about 750 extra prisoners, but when realignment kicked in and the state's overcrowding was reduced that contract went away, Ahern said.

San Mateo County won't owe anything if it doesn't send anyone over, Ahern said.

"There's no cost to them and no loss to us," he said.

Email Bonnie Eslinger at beslinger@dailynewsgroup.com; follow her at twitter.com/bonnieeslinger.