A modern automotive air conditioning compressor draws about 2HP continuous with an initial requirement of 3HP when the clutch closes. Also, these compressors are designed with variable vanes to minimize the "thump" when the clutch is electrically activated.
Older air automotive air conditioning compressors could require as much as 4HP and the clutch was designed to take 5HP, because engagement was real harsh on both components.
Mower PTO clutches as they are called, generally rated to 5HP even if the engine is much larger, because it doesn't take 5HP to spin a 20", 22" or 24" blade. Remember that the larger mower decks have more than one blade, so the blades themselves on those decks are smaller than even a 5 HP push mower (typically 26-34" depending on the mower).
The PTO clutch on farm tractors is another animal altogether, and really just a one-way clutch used to avoid the situation where you push the clutch in on the tractor to stop movement and the momentum of the mower blades (or other implement) continuing to turn the PTO shaft, keeping the transmission bits turning and the tractor moving. It works much the same way as a ratchet wrench works, and if you rotate them by hand you'll even hear the clicking.
I was thinking about this for my son's riding kart (see "backbone kart" thread), because with a 4-5HP electric motor, this clutch might be a perfect fit - and allow me to spin the motor up a litlte bit before engaging the clutch.
