A friend of mine had a concern as to the cost to keep the Christmas lights on during this holiday season. As it so happens, I have a specialty in electrical systems. So, how much does it cost?
Let’s assume 5 strands of 150 lights each, for a total of 750 lights. We’re going to be old school and use the regular “white” Christmas lights, not LED’s. Christmas lights use “about 25 watts per 50-bulb strand”[1][2]. 750 lights is electrically equivalent to 15 50-bulb strands, or 
Thus,
Remember, that’s if you left the lights on all the time for two straight months. If you left them on for 8 hours a day instead of 24 hours, that would be $6.48.
Similarly, if you only used 2 strands of 150-bulbs each for 8 hours a day, that would be $2.59.
Or, if you used 2 strands of 50-bulbs each for 8 hours a days, that would also be grand total of 86.4 cents.
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