I did an inspection today for a homeowner with high electric bills. He is gung-ho to start some monitoring, and had already researched Kill-A-Watt, TED, and similar meters. He has some motor loads, such as furnace blower, septic and well pump, water recirc pumps, etc. He asked about how accurate his results would be.
What I told him--and I hope it's true--is that if he used typical electricity monitors, he would get the same numbers for his motor loads that the utility is getting using their service meter, but that they would not be entirely accurate due to power factor. His cost figures should match theirs, but he wouldn't be getting actual watts consumed.
My understanding is that reactive loads can't be measured entirely accurately with typical inductance tools like the current clamps used with multimeters, Kill-A-Watt / TED type monitors, etc., and that an actual watt meter would be needed if one really wants to know exactly. A commercial or industrial customer with a lot of motors would care about this, but a typical homeowner would not.
How close am I?