How do you assumed power factor?

Power Factor is a maximum of one when the voltage and current in an AC waveform are in phase. Since power in watts is equal to voltage x current x power factor, you get maximum wattage with power factor = 1. This occurs with a pure resistive load. When you power an inductive load like a motor the power factor decreases and so does the delivered power. That is why motors are typically rated in VA, or volt-amperes rather than watts. So y Ou can assume power factor = 1 when you have a load measured in watts like a light bulb or space heater and perhaps in the .75 range for an inefficient motor.