CPS Energy will generate about half of its power from renewable energy sources such as solar or wind by 2040 under a new “flexible path” strategy presented to the board by CEO Paula Gold-Williams on Tuesday.

Designed to reduce the utility’s reliance on fossil fuels, environmental groups say the plan doesn’t go far enough and will reduce air quality by continuing the use of coal and natural gas power plants for decades.

Roughly 22 percent of CPS’ power currently comes from renewable energy sources, with its natural gas plants generating 45 percent, coal accounting for 18 percent, and nuclear making up the rest, according to the utility. Owned by the city of San Antonio, CPS has been trying to wean itself off fossil fuels in recent years.

“We will not be building a large base-load power plant again. I think we can say that pretty confidently,” Chief Operating Officer Cris Eugster said before the board meeting. “I think how we add capacity in the future to meet our load, our preferences is to do it with renewables, with solar, but we need to do it in a way that’s affordable for our community and in a way that we get reliable power.”

The utility plans to add 4,100 megawatts of renewable energy sources and 550 megawatts of battery storage over the next few decades, Gold-Williams said at the CPS Energy monthly board meeting. One megawatt can power between 200 and 300 homes in the winter, and 200 homes on a hot summer day.

Mayor Ron Nirenberg said he would oppose building more natural gas and coal power plants.

“There are alternatives to us for large-scale generation,” Nirenberg said in an interview last week. “Short of the population doubling overnight — which it’s not going to, we’re growing into it — the rise of conservation to the renewables to all these other changes are in support of less large-scale generation needed for a city our size.”

CPS added nearly 500 megawatts of utility-scale solar power through contracts with San Antonio-based OCI Solar Power between 2010 and 2017. An additional 88 megawatts of residential and commercial solar power has been added through rebate programs in CPS territory. The utility ramped up its wind-power generation between 2002 and 2012 when it added 1,059 megawatts of wind power. It hasn’t expanded its wind program since.

Executives said CPS won’t be able to fully wean itself off coal until at least 2042 under the plan.

“The plan presented to the board of trustees today shows that CPS Energy isn't leading on climate action, despite its involvement in the SA Climate Ready process,” Kaiba White, an Austin-based energy policy specialist for Public Citizen, said in a statement. “Instead, the utility plans to burn coal until at least 2042 and wants to invest in a significant amount of new natural gas generating capacity.”

Greg Harmon of the Sierra Club said the utility is “overlooking a golden opportunity to launch a serious clean-energy economy.”

“CPS Energy is putting off the acquisition of additional utility-scale renewable energy for far too long and is intent on relying on dirty coal through 2042 despite its public-health risks,” said Harmon, who is the clean energy organizer for the Sierra Club’s Lone Star chapter.

Eugster said the utility is still figuring out what the future looks like as it examines whether it will retire its three oldest plants — the V.H. Braunig units with more than 800 megawatts of capacity — near the end of the 2020s.

“Our belief is our traditional fleet is really our bridge into our future,” he said, referring to CPS’ existing fossil fuel plants.

He added that the current timetable means CPS will have until 2022 to decide what a new “flex gen” power generation would look like. The plan shows that generation would be needed starting in 2025.

“If energy storage comes through in a big way, that could be all solar plus storage,” Eugster said. “Paula has also talked about that might be small box (natural gas).”