The following formula calculates the power used by any electric device.
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)
Hours of use
(the Watt) (multiplied)