U.S. electricity demand is growing at twice the pace new supplies are being added, an imbalance that could lead to supply problems in a couple of years, an electric-industry group said.

In its latest 10-year forecast, the North American Electric Reliability Corp., or NERC, found supplies thinning fastest in California, the Rocky Mountain region, New England, Texas, the Midwest, and the Southwest.

NERC, which oversees the reliability of bulk power systems in the U.S. and parts of Canada, said peak demand is expected to increase 18%, or about 135,000 megawatts, for the U.S. during the next decade.