Actually both, when your MH start it's create a surge that may exceed the initial 15 amp, A ballast that start may get surge up to 40 amp for a very small moment, not enought to trip your household breaker, but with time will weld your powerbar in a open state permanetly.
Now the other part, which is a diferent mather completly for the previous one, is the continuous load, most household wall socket are protected by a breaker in panel that is rated for 15amp in most case, some 20amp but rarely. So if you try to pull more than 15 amp continuously alot of thing is going to happen to you. First your household wire and panel breaker will heat up alot, your panel breaker should also trip, but the ultimate worst case is if your breaker does trip because it's really old then thing start to worst, you wire can heat up to a point where it will become dangerous.
Anyone if him wrong please feel free to correct me!