Jim,
PTO horsepower is the power at rated PTO speed (540 rpm) measured at the PTO with the tractor not moving. Engine horsepower is given in 2 different forms: If it is measured at the flywheel with all of the "accessories" such as cooling fan, coolant pump, inlet and exhaust ducts, hydraulic pumps, etc. in place, the result is called Net or Flywheel Horsepower. In general for gear drive transmissions, PTO horsepower is approximately 85% of Net Engine HP. If the tractor is hydrostatic, the hydro pump always consumes some power even when the tractor is stationary, so PTO horsepower for a hydro is approximately 83% of Net Engine HP.
Sometimes, in order to make the number look higher for customer appeal, the horsepower loss from the "accessories" is added back to Net to get a number called the Gross Horsepower. It is not a meaningful number since not all of it is available for useful work. Manufacturers of compact tractors and lawn and garden tractors usually first quote the Gross horsepower and may also quote a PTO horsepower. Agricultural tractors from approximately 50 hp and up are usually quoted as PTO hp if a row crop tractor and Net Engine hp if an articulated 4WD tractor.
An entirely different power rating is sometimes specified for the gearboxes on implements such as rotary cutters. They often give a rating in HP which is the maximum continuous hp that can be delivered and not damage the gearbox. Usually this number is substantially higher that the PTO power output of the tractor that will operate the cutter to account for peak shock loads that the cutter gearbox will see in real operations. In other words, you could buy 5 foot wide rotory cutters with a range of gearbox horsepower ratings. Lower horsepower implies a lighter duty machine and higher implies a more ruggedly built machine. If your compact tractor were rated at 25 PTO hp, it would not be uncommon to find the lightest duty rotary cutter available that would cut the width of the tractor would have a gearbox power rating of 35 or 40 hp.
Often tractor manufacturers will provide guidelines as to the size of the implements recommended for each tractor they sell. Normally an implement manufacturer indicates required horsepower in terms of PTO horsepower, but they often omit actually stating the term "P T O". So to get to your original question, you would usually be safe in assuming that implement power requirement ratings (not gearbox) match with tractor PTO power ratings.
Good Luck.
JackIL
