As a former science teacher, I always enjoy putting pencil to paper to mathematically predict what a season of festive lights will cost me. This year I decided to share my calculations so you can do the same with your light display if you want to. I will only show the calculations for the lights on tree pictured here so I don’t overwhelm you with figures and formulas.
We have two types of lights on our 8’ artificial spruce tree each year. The smaller white lights are LED with 50 bulbs per string and use 2.4 watts per string, or .048 watts per individual LED bulb when lit. We have 6 strings (300 LED bulbs) on our tree each year. This is how much electricity is used by the small LED bulbs:
300 LED bulbs X .048 W/bulb X 16 hr/day = 230 Wh/day
Knowing that electricity is measured by kilowatt hours (kWh) and that the average price per kWh is $0.133, we can calculate the cost of those 230 Wh of electricity per day and for the 37-day holiday season.
230 Wh/day X $0.133/1000 Wh = .03 cents/day $0.03/day x 37 days = $1.11 for the season
Now to calculate the cost of the larger C7 color bulbs on our tree each year. These are the larger incandescent lights that we have used for many years. We use three strings of 25 bulbs each. Each bulb uses 7 watts of electricity totaling 175 watts per string, or 525 watts for all three strings. Time to calculate how much electricity the larger bulbs use each holiday season:
75 C7 bulbs x 7 W/bulb x 16 hr/day = 8,400 Wh/day
Calculating the cost of those 8,400 Wh/day at $0.133/1000Wh gives us a total cost per day. And then multiplying for the 37-day season gives us a total cost for the 3 strings of C7 incandescent bulbs.
8,400 Wh/day x $0.133/1000 Wh = $1.12/day $1.12/day x 37 days = $41.44 for the season
You can see we spend over $42 per holiday season to light the family tree. Looks like it’s time to consider LED colored bulbs to replace the older C7 bulbs, since they are almost 40 times the energy cost! Do you think my wife would notice if I switched them out without consulting her? I think I’ll stick to the themes of peace and joy for the season and wait until next year to recycle the old lights! Happy holidays from the Educators in Residence at Trane!
