NEW HAVEN — The United Illuminating Co. has told state utility regulators that its plans for building “peaking” power generators involve spending $510 million to build the units at existing power plant sites in Milford, Middletown and Montville.

The peaking generating units — essentially jet engine turbines that would provide power only during the hottest and coldest days each year — would be built in 200-megawatt clusters at Milford’s Devon Station and the Middletown Station power plant, which is near the Connecticut River. Another 100 megawatts of peaking power would be installed at the Montville Station power plant.

“There’s a need for these kind of resources to keep the system (the state’s electric transmission grid) stable,” said Tony Marone, UI’s vice president of client services.

All three power plants where the peak units would be built are owned by NRG Energy, of New Jersey. NRG is UI’s partner in a new limited liability company called GenConn Energy, which has been created to bid into a process designed to fulfill the state’s need for generation units to operate during peak usage periods.

The state Department of Public Utility Control is reviewing numerous submissions from companies seeking to fulfill the 500 megawatts of peak generation expected to be needed. Regulators will announce the proposals that have been selected in June. The impetus for the submissions is a state law that was enacted last year.

Marone said the partners have structured their proposal so that the DPUC can choose to accept the entire bid or smaller elements of it.

“We’ve broken it out into 100-megawatt increments all the way down to 200 megawatts,” he said. “We don’t feel we could do any less than 200 megawatts because below that, the cost per kilowatt would just be too expensive.”

If the DPUC selects the entire UI proposal or any part of it, Marone said the peak generating units would be up and running by June 2010.

Connecticut’s other large electric utility, Connecticut Light & Power, of Berlin, has proposed building two peaking units of its own: a 200-megawatt plant in Lebanon and a 65-megawatt plant in Waterbury.

Milford Mayor James Richetelli Jr. endorsed the UI plan.

“Two or three years ago, we were concerned that Devon Station might be closed altogether,” Richetelli said. “The land is already being used as a power plant and we’d be getting rid of two outdated dirty units that were built in 1956. This is going to help Milford on many levels, not the least of which is increased tax dollars and new jobs.”