Question: how to figure out how many watts something uses?
Answer: The number of watts is equal to amps multiplied by volts. That's it! ... 
For example, if the current is 3 amps (3A) and the voltage is 110V, you multiply 3 by 110, to get 330W (watts). The formula is P=3A X 110V = 330 W (with P standing for power).
This is why watts are sometimes called volt-amps.