Rotary hammers, how many joules to hammer through concrete slab?
It's mostly going to be about speed - if you are patient and like handwork, a star drill and 4 lb / 2 kg hand sledge will drill holes in concrete or granite. If you plan to drill many small holes and then hammer out the plug, the smallest rotary hammer drills will generally do that just fine. The bigger ones may do it much faster, and will do it with bigger bits. 

The "nice way" to drill finished holes of the size you want is with a core drill (which just drills around the edge, like a hole saw, leaving a plug or core) but those cost serious money even to rent/hire, usually.