The "peaking" power plant will be important for SRP to quickly meet electricity demand on hot summer afternoons or when other power plants unexpectedly shut down, said Alex Pourbaix, president of TransCanada's energy division.
"The 12 units offer a great deal of efficiency and flexibility," he said.
The plant will be able to supply as little as 20 megawatts of electricity at a moment's notice, which could supplement power lost if the breeze dies down at a wind farm supplying SRP, or move to full power if a major electrical source powers down.
"You never know when you might need the plant," Pourbaix said.
"It could be the middle of December and somewhere else there is a problem, and you need the power on very short notice."
The plant is expected to run 400 to 700 hours a year, or less than 8 percent of the year.
SRP officials declined to say what they will pay TransCanada to keep the plant ready to switch on or to provide electricity.
The utility also is building two of its own natural-gas-burning plants in the region, one that will offer similar peaking abilities and another that can run for longer stretches.
An 850-megawatt peaking plant is planned north of Florence, and a 1,150-megawatt intermediate plant is planned east of Casa Grande, but because of the slowdown in customer growth and demand, both have been delayed.
SRP also was considering a fourth natural-gas plant near Eloy, but the need for that facility also has diminished, officials said.
"We have to watch the economy and see how that plays out to make a final decision on when to bring those (plants) in," said Charlie Duckworth, SRP's manager of energy management and information.
The peaking plant near Florence could be built in phases, beginning in 2013, and the intermediate plant near Casa Grande may not be needed for a decade.
SRP isn't doing anything to actively develop the Eloy site but could revisit the project if demand requires it, Duckworth said.
So far, TransCanada's Coolidge site has been graded, with construction scheduled to begin soon.
More than 200 people are expected to work on the project, and about 13 will be needed to run the completed plant, Pourbaix said.