ERCOT's Seasonal Assessment of Resource Adequacy, or SARA, report had projected that peak demand on the hottest day of summer would probably reach around 77,300 megawatts (one megawatt is enough to power about 200 homes on a hot day). But Jones said forecasters did not anticipate temperatures to get close to where they were during the record heat logged in 2011. Temperatures that summer were about 5 degrees hotter than average.

"The question we always ask of our weather forecasting team is: Is this 2011? And you’d hear no, this is not 2011. Everyone was saying this is not 2011. And about the end of May they’re saying, this looks like 2011. And at about the end of June, they're saying it's like we're heading into 2011," Jones said.

He said the extreme heat scenario for this summer estimated that demand could reach above 81,000 megawatts, which was the original forecast for Monday. Forecasters originally gave that scenario a 5 percent chance of happening, he said.

"The 81,500 (megawatts) was the extreme, but those numbers are in the system. We just start with what we think the expected case is, and in early May, the expected case was 77,500" megawatts, he said.