eNotes editors are only allowed to answer one question at a time. In physics, power is defined as energy per unit time and is measured in watts. Since energy is measured in Joules and time in seconds, the units for watts are J/s. A kilowatt (kw) is 1000 watts and is a more convenient measure for larger amounts of power. So why do we measure electricity usage in kilowatt hours (kwh)? Because we need to know not what the total amount of electricity used is at any given moment in time, but how much electricity is being used over a given period of time. So the kwh is used to measure the amount of power you use over a given period of time. So each electrical device in your home consumes a certain amount of wattage while it is turned on. You are charged for not only the amount of wattage it consumes at any given point in time but also the amount of time that it consumes the power. That is why there is a time factor in measuring power consumption.