The Watt equals one joule per second. For example, if a 60W lamp remains on for one hour, the power consumed is 60 watts-hour. This would be the same amount of energy that would make a 120-watt bulb shine for half an hour. To get an idea of how much Watt represents, here are some reference values of the power of some household appliances: stereo, 200 watts; vacuum cleaner, 1000 watts; computer, 400 watts; exhaust fan, 300 watts; electric oven, 5000 watts; hair dryer, 1300 watts; LCD television, 150 watts. The name of this power unit is a tribute to the British engineer James Watt, for the recognition of the many researches that he made, to better understand the energy, as well as, the way in which he created machines that obtained a better use of it.
The Horsepower came about when, for the first time in history, people had something like power to use other than animal traction. This "something" was a steam engine and James Watt pioneered the construction of these steam engines. His target audience was precisely the people who used horses to do the heavy work, which would then be done by machines. Sales did not go well at first, so to help sell the machine it was necessary to equate the work developed by the machine with the work done by a horse. That is why the horsepower measurement unit was created. To be more precise in assessing this unit of measure, James Watt has carried out various experiments in order to measure the work performed by a horse for a certain period of time. From there he just needed to see how long it took his steam engine to do the same job.