Factoring for the efficiency factor

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I found an article here which explains the WAV formula well and gives several good examples. It also has a rather confounding notion it pushes which states that though watts equals amps times volts, a kilowatt is not amps times volts divided by 1000. It says that there is a factor called the efficiency factor which accounts for the inneficiencies of the equipment. The number is almost never known for any device, varying from 65% and 95%. The author recommends to assume 85%, that it will be close enough for UPS assessment. I really have to wonder how one goes about ascertaining the efficiency factor of a given piece of hardware.