NEW ORLEANS — Susan Guidry stepped up as a volunteer in New Orleans after Hurricane Katrina, helping clear debris from the streets as part of a group calling itself the Katrina Krewe. She saw firsthand the disaster’s toll, including the crippling of the power supply. When voters elected her to the City Council, she said, she hardly knew what a kilowatt was. But she came to the conclusion that the city had to change its approach to energy.

“As fragile as New Orleans is with climate change, hurricanes, sea-level rise, I just started researching,” Ms. Guidry said. “That was a lot of hard learning.”

What she found out led her into battle over a question central to the climate debate. Is it wise to keep building fossil-fuel plants — even those powered by natural gas rather than coal — that will be in operation for decades? Or are wind turbines and solar farms now reliable and economical enough to take their place?

Ms. Guidry began her homework after a subsidiary of Entergy — a major utility in a state heavily reliant on the oil and gas industry — said it needed to build a new natural-gas plant to replace an outdated unit in the New Orleans East neighborhood. When the issue arose in 2015, “that probably sounded fine to me,” said Ms. Guidry, whose district hugged the city’s western boundary. “There was solar power, there was wind, whatever. It all seemed a bit ahead in time for that to be sufficient for us.”