First, find the total amount of bulbs in the four sets of Christmas lights. Then, multiply that number by the power of a single bulb to find the total power of the four sets combined. Then, multiply the total power of the four sets by the time that they remain active to find the energy that they will consume.
Since there are four sets of lights each holding 25 bulbs, there are 100 bulbs in total. Since each bulb has a power of 7W, then the total power of the four sets combined is 700W.
On the other hand, each day the bulbs are lit during 4 hours, and there are 41 days from December 1st to January 10th. Then, the lights are lit during a total of 164 hours. Since there are 3600 seconds in one hour, the total amount of time during which the bulbs are lit, is:
Since the total energy E delivered by a system with power P during a time t is given by the equaiton:
Then, the total energetic cost of running the lights during that period, is:
Therefore, it costs 413 million Joules to run the lights during that period.