The US Middle Atlantic and Southeast states experienced five days in a heat dome from June 23-27, 2025. This extensive heat dome stressed the electric generation, transmission and distribution resources of the region, including those of the Midcontinent Independent System Operator (MISO), another of the largest ISOs in the US. MISO issued an emergency declaration in anticipation of the effects of this heat dome on demand and consumption, requiring all generation resources to be available for full capacity operation throughout the period, as displayed on the MISO Live Dashboard.
The chart below summarizes the Midwest Independent System Operator (MISO) experience during the heat dome. Peak generation was approximately 120 GW. Nuclear generation remained constant throughout the period. MISO adjusted both coal and natural gas generation output to match demand. Solar made significant contributions on all days. Wind made significant contributions on days one, three and four. MISO also took advantage of electricity imports from adjacent utilities on all five days, as well as undefined “other” resources.
The graph below summarizes the MISO experience in mid-September. Peak generation was approximately 106 GW, or approximately 88% of the peak generation during the heat dome. Nuclear generation remained unchanged at approximately 10.75 GW. MISO again adjusted both coal and natural gas generation to follow peak load. Solar made significant contributions on all days, with maximum generation of 13.25 GW on day two. Wind contributed on all days, though unpredictably, with the most significant contribution on day one at 13.55 GW. MISO also took advantage of electricity imports from adjacent utilities on all five days, as well as undefined “other” resources.
The graph below shows the hourly generation on September 15th. Peak solar generation was approximately 12.5 GW. Peak wind generation was approximately 13.55 GW. Coal generation varied from 17.5 – 40 GW and natural gas generation varied from 26 – 40 GW, both in load following mode.
If the peak solar generation of 13.25 GW on day 2 is the rating plate capacity of the MISO solar generation assets, then the solar capacity factor on September 15th was approximately 32%. If the peak wind generation capacity of 13.55 GW is the rating plate capacity of the MISO wind generation assets, then the wind capacity factor on September 15th was approximately 40%.
MISO operated through both the June heat dome and the September period with the availability of a large coal generating station which remained in operation as the result of a presidential emergency declaration which is set to expire at the end of the year, though it might be extended if the emergency persists. MISO has apparently not taken any coal generation out of service for scheduled maintenance since the heat dome.
MISO remains far from displacing coal and natural gas generation with solar and wind; and, even further from replacing coal and natural gas with solar, wind and storage.
Originally published here.