Imagine the amount of solar they could put on the roof tops of those warehouses to offset the costs
And does Amazon own the warehouses outright, or do they lease them?
Why not just double your fleet size instead?
That's a bit extreme, but this is the kind of optimization that your bean counters are good at. You probably don't need to double your fleet but just keep enough that you can shift the consumption curve or take advantage of price changes.
(Not private thank you very much)