Pretty sure the UK already ran into this problem when the North Sea had low wind for a prolonged period. This year wildfire smoke would have cut into the valuable days for many solar installations in the northern US/Canada, many hazy periods were close to a week. The intermittency problem is only going to get worse as the amount the grid relies on grows, it's easy to cover the occasional 5% loss grid wide, but a 25% loss is a big problem.