Different generators have different heat rates. There are also operational considerations (turning off a combined cycle isn’t flicking a switch). Further, the

Different generators have different heat rates. There are also operational considerations (turning off a combined cycle isn’t flicking a switch). Further, the two-settlement system means you are guaranteed the day ahead price which can be higher than spot rates. Furthermore, in the US you can trade the spread between day ahead and spot. Therefore, it isn't always rational to shut off when spot gas is higher than electricity times your heat rate.