Is "switch on" really the problem? As Germany showed during the solar eclipse (https://www.dw.com/en/german-power-net-survives-solar-eclips...), as long as it's a predictable event, you can have the necessary generation already "switched on" and running at a low power, and the limit becomes only how fast you can ramp up their power. Which is still an issue, but not as much as it would be if they had to turn on all the gas generators with zero notice.