There's lots of issues here. One is simply that people don't submit jobs during non-working hours, and most jobs don't take more than 4-8 hours, so unless you have a pile-up of jobs during working hours, the grid tends to sit idle overnight and on weekends. Another problem is that you plan your grid capacity for peak periods (and add a margin for future growth), so it will generally be underutilized. Another issue is that corporations create redundant grids for business continuation purposes, so often you'll have full grids that are hardly utilized at all.