Kilowatts (kW) is a measurement of power requirements of an appliance. Kilowatt-hour (kWh) is a measurement of power usage in terms of kilowatts per hour. For example, a computer might have power requirements of 400W. Every hour that it is running will use up 400 watt-hour. That is 400Wh. If the system is left running for 5 hours, the total power usage is 400W x 5h = 2000Wh or 2kWh. This is also how the power utility company calculate your monthly electricity usage and bills you accordingly.