Multiple U.S. power grids are facing unprecedented strain as a prolonged heat wave pushes electricity demand to record levels, prompting grid operators in Texas, California, and the Midwest to issue conservation alerts.

ERCOT the Texas grid operator asked residents to reduce electricity usage between 3-8 PM for the fifth consecutive day as demand approached the grid maximum generation capacity. The grid narrowly avoided rolling blackouts twice this week.

California ISO issued similar warnings, noting that solar generation drops off just as evening air conditioning demand peaks. The state is relying heavily on natural gas plants to fill the gap.

The heat wave now in its third week has been linked to a persistent high-pressure system parked over the central United States. Temperatures have exceeded 100 degrees in cities from Phoenix to Chicago.

Climate scientists say the increasing frequency and intensity of heat waves is straining grid infrastructure that was designed for a different climate. The Infrastructure Investment and Jobs Law included billion for grid modernization, but much of that funding has yet to reach projects.