If, for a large motor, you are given cumulative kWh values at 5-minute increments, how would you determine an instantaneous power consumption value?
You can determine average power consumption (kW) because it is kWh divided by hours.
Instantaneous power varies wih the cyclic nature of the voltage (v) and current (i) waveforms and, the instantaneous product of v and i is "averaged" in order to determine average power from which kWh is derived.
Note also that for many AC systems, the phase angle between v and i is important to consider: -
But, you can't assume the load is purely linear and, because of this, the instantaneous power may be very much higher in certain places than the average power value would imply.