Watt-hour / kilowatt-hour / megawatt-hour

These are units of energy (an amount, not a rate): a watt-hour is the energy delivered by one watt of power sustained for one hour. A kilowatt-hour (kWh), the unit that shows up on a home electricity bill, is 1,000 watt-hours; a megawatt-hour (MWh) is 1,000 kilowatt-hours. The distinction from the power units is exactly the energy-vs-power distinction: a 1 MW plant running for one hour produces 1 MWh of energy.