They might coordinate with TVA for transients (i.e. going from an idle machine to a full-machine run), but in normal operations these systems are at least 50% full. In my experience as a user these machines are more than 80% most of the time. I don't have hard numbers on the average utilization over the last week/month/year (these might even be classified), but you don't by and build such a machine to be idle.

I could find the numbers for two German super computers I have used in the past. SuperMUC (Phase 1 and 2 combined) had "above the desired 85%" utilization in 2017 and Hazel Hen at HLRS in Stuttgart reported a utilization "between 92% and 98%" in 2017.