The company, which is part of Alliant Energy, says the region set a new peak of 3,701 megawatts at 6 p.m. on Monday.
Company president Tom Aller says the hot and humid weather has taxed its grid, adding that its resources will continue to be tested until conditions cool off.
The company says Tuesday's usage could match Monday's record.
The previous high was set in August of 2003.
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