most ballast dont burn or run like using a icecap ballast, you can end up in a couple situations, A the bulbs will have a considerable shortened life span, B can over drive to the point of just burning out or causing the bulb to pop, C might work for a short period and cause a fire,
alot of ballast will be marked if your able to run them at different wattage, example alot of the ballast that are for 39w t5 can be used on 24w t5 but the wire config is different and can run more 24w then 39w, if un-certain list the exact ballast in it and read what it says on the ballast, most will be specific in wattage and linear running