The following formula calculates the power used by any electric device.





For example:

An ordinary light bulb, (100Watt) uses in 1 hour:
100Watt * 1 Hour = 100Wh (Watt Hours)

While an "economical" one of 15 Watt uses in 1 hour:
15Watt *1 Hour = 15 Wh (Watt Hours)





Power                     *             Hours of use      =    Energy
(the  Watt)       (multiplied)