Rapid Tables, Convert To and Kyle's Converter offer online calculators that convert watts to BTU. On each website, the user enters a number of watts and the equal amount of BTUs per hour automatically populates.
Under the online calculator, Rapid Tables explains the formula for converting watts to BTUs per hour. The website states that one watt equals 3.412141633 BTUs per hour. This number, multiplied by the number of watts, derives the desired conversion.
Convert To allows the user to specify the what precision is wanted for the answer. The website explains that the precision number is the number of digits shown after the decimal point. Kyle's calculator also allows the precision of the answer to be specified. Additionally, this website explains the that a watt is equal to one joule of energy per second.