Less solar, more coal: How Duke kept power on in a heat wave

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Less solar, more coal: How Duke kept power on in a heat wave

Published: Wednesday, August 20th, 2025 @ 9:58 pm
By: John Locke Foundation ( More Entries )

    Publisher's Note: This post appears here courtesy of the John Locke Foundation. The author of this post is Jon Sanders.

    Early in the morning of June 24, amid a blistering heat wave affecting most of the nation, the U.S. Department of Energy (DOE) granted a request from Duke Energy Carolinas by issuing an emergency order authorizing Duke to exceed environmental emissions levels if needed to dispatch enough electricity generation for maintaining power provision during the extreme heat.

    The order by Energy Secretary Chris Wright acknowledged the unusual combination of heat and humidity in Duke's service area from June 23 to 25, "with heat indices in the range of 100˚F to 110˚F," that would lead to "unusually high" electricity loads. The emergency order helped Duke maintain "maximum reliability on its system" and "identify and dispatch generation necessary to meet load requirements" to avoid "curtailments and outages, presenting a risk to public health and safety."

    The order authorized Duke to obtain power generation from additional generation units "regardless of emissions or other permit limitations" through 10 p.m. on June 25. Duke's request had anticipated possible "exceedance of emissions of nitrogen oxide and particulate matter."

    The following table and graph shows how Duke's power generation from different sources changed from the day before the emergency order (June 23) to the last day of the order.

    Duke Energy Carolinas daily electricity generation before and during the June 24-25 heat wave, by source (MWh)

Here are several takeaways:

Nuclear - a zero-emissions resource with a 100 percent productivity factor in North Carolina - provided unwavering power, generating nearly half the production each day: 174,898 megawatt-hours (MWh) on June 23 and 174,581 MWh on June 25, almost identical amounts.
Solar provided very little power each day (obviously only during daylight hours). Duke used 20 percent less solar generation after the order - only 7,192 MWh (1.8 percent of total generation needed) on June 25.
Natural gas provided the second-highest amount of generation, and after the emergency order, Duke increased its utilization by 6.2 percent.
Coal was the big mover - after the emergency order, Duke increased its utilization of coal by almost 30 percent.

Crucially, no outages or blackouts were reported. Duke was able to keep the power on by shifting away from solar generation (20 percent less) and more to natural gas (6 percent more) and especially coal (30 percent more). Duke required about 27,000 more MWh of electricity on June 25 than on June 23, and most of that was provided by additional coal generation.

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