This is a short but very clearly articulated opinion on motor hp. The author Peter Scott, if he is not a teacher, he missed his calling. The very short, but straight to the point explanation of "power factor" (a concept understood by few) is clearly communicated. My college instructor didn't come anywhere near this clear an explanation.
100% efficient electrical usage runs at a power factor of 1 or "unity". Resistance circuits such as heaters run at a power factor of 1, all of the incoming energy is converted to heat. Motors are different, Peter explains it well.
Case In Point: I have talked about operating the power house at Port Arthur Shipyard before. Much of the equipment in this power house was still original from the pre-WW1 era. To save on our energy bills the power house monitored "power factor". Induction motors have poor power factor, synchronous motors run very close to unity. When we were running all three 300 hp AC to DC converters, to supply the dry dock and foundry with DC power, the big induction motors would pull the power factor below .85. At that point we would shut down one of our induction motor compressors, and start a 300 hp synchronous motor, running a Joy compressor. The synchronous motor would bring the plant power factor back up to .9 to .92, from .80 to .85 saving the shipyard a good amount of money on their hydro bill.
So without further ado here is that short but well communicated article on the subject matter.