A watt is defined as 1 joule per second and is therefore a measurement of energy transfer over a period of time.
A kilowatt-hour is a measurement of energy equal to one kilowatt sustained over a period of one hour.
What is the point in double counting units per time? It seems to me that a kilowatt-hour could be more simply expressed as 3.6 megajoules.
This question was prompted by this article which triple-counted the time unit (using seconds, hours, and days): "New York City [uses] about 12 gigawatt-hours of electricity a day" Would it not be simpler and more intuitive to just state: "New York City uses 43.2 terajoules per day" or "New York City uses 500 megawatts"?
Is there a justification for using kilowatt-hours or is it used for historical reasons?