A friend of my 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 . The base rate for electricity in Seattle^{3} is 3.86¢^{4} per kWh^{5}.

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.

Point being, it doesn’t cost all that much to run your Christmas lights. It costs an order of magnitude less if you use LEDs, which use about 0.05 to 0.07 watts per a bulb. Move the decimal one place to the left on the above costs and you have a pretty good idea of what it costs to run an equivalent^{6} LED string of lights.

And that’s how engineering works, folks.

NB: I’m experimenting with using to display equations. It’s really quite spiffy.

0GreenOptions reports consumption for each mini-light at about 0.5 to 1 watt ↩

http://www.seattle.gov/light/accounts/rates/docs/2009Oct_rsc.pdf ↩

assuming you’re a residential user and use less than 16kWh/month ↩

kWh is a funny measurement and is actually

*not*time based. For example, a 100 watt bulb that is on for one hour uses 100 watt-hours of energy. Similarly, a 100 watt bulb that is on for 30 minutes uses 50 watt-hours of energy. ↩by number of lights, not output of light ↩