PJM Interconnection LLC has declared a level one emergency for the 13-state eastern US grid and called on all power plants to operate at full capacity Thursday during a heat wave.

The largest US grid issued an Energy Emergency Alert level one, meaning it is concerned about being able to maintain adequate power reserves on July 27 as consumers and businesses turn up their air conditioners amid scorching heat. PJM has ordered all power plants to be online and for consumers enrolled in demand-response programs to be ready to curtail their electricity usage to keep power supply stable, according to a notice late Wednesday.