The power distribute to a 15 watt light bulb in normal operation is 15 watts. In normal operation how greatly energy is delivered to such a bulb in an hour?
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From the definition of power one must be able to reason that E = Pt where E is energy P is power and t is time. If P is how speedy energy is being delivered to the resistor that is if P is how many Joulesper second of energy are being delivered to the resistor then of course all one has to do is multiply by the number of seconds the time to get the total energy delivered in that time. Currently an hour is 3600 seconds (not 60 seconds!) so E = (15 watts) * (3600 seconds) which means E = 54,000 joules which is the similar as 54 kJ.