Answer:
The units are not interchangeable: a kilowatt is a unit of power, and a kilowatt-hour (one kilowatt applied for one hour) is a unit of energy.
Example:
A 100 watt bulb is, at any given time, using 0.1 kilowatts of energy. Using a 100 watt light bulb for one hour consumes 0.1 kilowatt-hours of electricity. Energy is only used by applying power for a finite period of time.