# I Got 99 Light Bulbs, but My Decorations Ain’t Done

Confession time: I really dislike winter. Like, really. I despise the cold. All of my life I have experienced winter, and it wore me down to such an extent that I legitimately planned all throughout college to move to a warmer climate. Instead I ended up moving to Boston. And this past winter, Boston broke its personal record of snowfall with 110.6 inches. For those of you who live anywhere but the United States, that amounts to

In other words, a lot of snow. So apparently I moved to the wrong place.

But there is one thing that makes winter worthwhile. Christmas. Once the New Year comes along, I wish the weather would start heading toward spring immediately. But until then, Christmas cheer gets me through. In fact, I actually prefer cold weather during Christmas.

So yeah, I like Christmas. I like Christmas so much that this is the second year in a row that I’ve written a PTC Mathcad blog centered on “the most wonderful time of the year.” And in last year’s Christmas-themed blog, I jokingly listed out several ideas for topics revolving around PTC Mathcad and Christmas. As it turns out, they weren’t all terrible ideas. Therefore, I’m going to tackle one of those ideas this year: figuring out how much electric energy is used each year due to Christmas decorations.

Okay, so we won’t be able to find any sort of exact number. Rather, I want to figure out some kind of minimum. But first things first: I need to define some new units.

These will come in handy a bit later on. I should also mention that we’re only looking at usage in the United States. Getting data about holiday behavior from multiple countries would be extraordinarily difficult, if not impossible.

The U.S. population as of July 2014 was 318.8571 million. According to 2010 Census data, the average household size is 2.58 people (sounds painful). If we assume the average household size is roughly the same, we can estimate the number of households in the U.S. by dividing the population by household size.

According to a 2010 Gallup poll, 88% of Americans put up a Christmas tree for decoration and 87% put up other decorations. So we can estimate the number of households in the U.S. that have Christmas trees and other Christmas décor.

Right now in my office I have a 7.5 foot Christmas tree. On that Christmas tree, I have two strings of lights, each with 100 bulbs. According to the information on the tag, each bulb is rated at 0.425W, which should give each light string a rating of 42.5W. However, the tag says it’s rated to 40.8W, which would be accurate for 99 bulbs, so I’m thinking there might be some false advertising going on. I’ll need to count the light bulbs myself when I take the tree down. But that’s another issue!

Back to the point, I think it’s pretty fair to say that if a household is putting up a Christmas tree, there would be at least two light strings with at least 100 bulbs each. I don’t think I’m stretching too much here. Moreover, if a household is putting up other decorations, we can safely assume that that additional decoration incorporates at least one light string with at least 100 bulbs. Remember, I’m trying to find a minimum here. I am not accounting for moderate to extreme decorations, like you might find at the Johnson home.

If we assume that each light string has a power rating of about 40.8W, we can compute the power used for Christmas trees and for other decorations. Multiplying that power by the number of households that put up that type of decoration, we can calculate the total power.

Over 13 gigawatts! It only took 1.21GW to send Doc Brown and Marty McFly back to the future.

Time travel aside, that’s a tough number to understand. Let’s move forward a bit. I would say that the vast majority of Christmas decorations are put up around Thanksgiving, and they probably hang around into mid-January. But to be on the safe side, let’s say the decorations are up for 30 days. In many cases, those lights are likely on 24/7, but let’s be conservative and say 12 hours per day. Multiplying that time by the total power gives us the total energy consumption, which I’ve displayed using the kilowatt-hour unit I defined at the beginning.

The average price per kilowatt-hour in the U.S. is 12 cents. Using that, we can calculate the total cost.

More than half a billion dollars! Now remember, that’s a minimum. Most houses are putting up significantly more than one or two light strings. And then you have businesses and parks and department stores with hundreds of light strings. At a bare minimum, Americans spend over half a billion dollars each year simply to power their Christmas lights.

It’s beginning to look a lot like Christmas.