Salesmen like to talk big numbers, so they usually use the gross HP which is what the naked motor (no fan, radiator, alternator, etc.) will produce in a test stand. Net HP is what is available at the flywheel after all that aforementioned stuff is added. There are further losses in the transmissions (hydro being less efficient than gear) hydraulic pumps, etc. PTO HP is the power available to do work at the PTO and is the most important and invariably is less.
Manufacturers of chippers usually specify a range of PTO HP desirable to run their chippers. The low number may be a little optimistic; the high number probably allows continuous feeding. Another factor is the
chipper flywheel weight. "Better" chippers (Wallenstein, Salsco, Valby, et al) likely have heavier flywheels that can store more energy than lesser machines. As a retiree you may be willing to feed more slowly or wait while the engine builds up speed after it has bogged down. Gravity-fed chippers are more efficient than hydraulically-fed chippers as there is no hydraulic mechanism to suck more power. Read many of the threads about chippers on this forum to get some feel as to the religious arguments about the merits of both types of feed and their effects on the person operating the
chipper. Yet another factor is the feed (not the hopper) opening. My tractor is at the lower end of the recommended power for my
chipper's size, but I can feed crooks, forks, and knotty branches through the large opening and am willing to let the engine recover if it bogs down.
Diligent reading of many of the
chipper threads will reveal many anecdotes of advantages and problems; read a lot of the threads to get an adequate sample.
