How many joules of energy does a 100-watt light bulb use per hour?

Watt is a unit of power that is a simplified form of the quotient of joules and seconds. 
  
                                    1 Watt = 1 joule / second

It is known that 1 hour is equal to 60 minutes that is also equal to 60 seconds. Converting 1 hour to second,

              1 hour x (60 minutes/1 hour) x (60 seconds/1 minute)  = 3600 sec

Thus, for every hour, there is 3600 seconds.

                                   Energy = (100 Joule/second)(3600 s)
                                   Energy = 360,000 joule

Thus, the energy emitted by the bulb is equal to 360,000 joule.