The US Department of Energy has warned of a heightened risk of power outages across a wide stretch of the country on Wednesday as an intense heatwave pushes electricity demand to near-record levels. The concern centres on the PJM Interconnection, the largest US electric grid, which supplies 67 million people from Washington DC to Chicago, prompting energy secretary Chris Wright to authorise emergency backup measures to help stabilise the network and avert rotating blackouts.

Late on Tuesday, Wright approved PJM's use of backup generation as a last resort ahead of a possible level three emergency alert, while the grid operator issued a "maximum generation emergency" alert asking generators to report how quickly offline units could return to service. Temperatures in Chicago, St Louis and Washington DC were forecast to exceed 90F (32C) at peak demand, while the Midcontinent Independent System Operator, covering 15 midwestern and southern states, predicted demand nearing its all-time high of 127.1 gigawatts and raised its reserve buffer accordingly. In rural Missouri, one electricity cooperative urged customers to unplug unused appliances and raise thermostats to ease pressure on the system.