I think it has to do with the availability of electrical power from the grid.
I believe I've seen where this draws so much power that it has to share with the surrounding area that it has to coordinate with them.
And it seems to me that there will be so much routine maintenance that they may as well schedule the shut-downs for the times of limited power availability.
I know some large paper mills and other factories which have to undergo periodic shut-dows for maintenance try to schedule it for times when power is the most expensive.
Arf
