Austin discovered this was star axis weakness

From the text:

“Water! They built protection around the power plant but not their water source.” He jumped up. “I know how to shut them down.”

“To power two million homes, their plant would have to create nearly twenty billion kilowatt hours of electricity. That’s twice the energy of a standard nuclear power plant. Given that the Elgen’s power creation is three times more efficient than a steam-turbine system, I’m guessing that the rats are generating heat close to one thousand degrees Fahrenheit. That’s why they built the plant next to a river—without water the bowl would melt down in a matter of minutes. Or even if it didn’t, the heat would kill the rats. No rats, no electricity. No electricity, no lights, no cameras, no electric locks, and no electric fences.” Ostin grabbed McKenna’s hand and pulled her to her feet. “Come on, we’ve got to tell the others.”