My occassional use treadmill has a 2.0 HP motor and haven't given me any trouble or felt strained. I don't really push it too much in terms of speed, time (usually 45-60min max) or elevation. I prefer to do my hard workouts outside and the treadmill is just to get a light workout in when weather is lousy outside.. The more HP the generally quieter it will be. I read that a 2.5 HP treadmill is a good choice for average weight runners, but I remember the treadmill salesman mentioning anything more than 2.0 HP is a waste since household current is already at capacity.