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In physics the unit of measurement of energy is called a joule. More commonly we refer to the energy or joules we use per second as Watts.Therefore a 20W light bulb is using 20 joules of energy every second.
On our utility bills, however, we don’t find consumption measured in joules, but in (kilo)watt-hours. A watt-hour is the unit of measurement used commercially because it’s easier to quantify. It’s the equivalent of the total energy supplied if electrical power of one watt is maintained for one hour; that’s equal to 3600 joules. So you’ll usually hear people talk about kilowatt-hours, if we’re looking at a utility bill for domestic use, or megawatt-hours if we’re looking at the consumption of, or energy produced by, a power plant.