But it's somewhat wasteful to build an entire industrial plant that's only run at 10% capacity

Generally not, no. Most manufacturers would rather turn it off as needed rather than turn it on as needed. Unpredictable outputs require unpredictable staffing rosters, introduce more risk into plant operations and does not give confidence to customers ("we need to delay your shipment").

Desal would need very big reservoirs to be able to erratically run, but perhaps that is done off peak in some places? Aluminium is complex, you can't let it cool too much otherwise you risk the whole process solidying (no recovery, requires rebuilding entire smeltery).