So Many Metrics, So Little Cents

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Continuing on the theme of the inadequacy of units of measurement, a couple of Duke professors recently revealed in their paper, “The MPG Illusion,” a very powerful misconception in the way we Americans look at fuel efficiency. In this post I’ll discuss their conclusions and discuss the repercussions for the bigger efficient energy usage picture.

To summarize the study, Professors Richard Larrick and Jack Soll conducted surveys that asked folks to choose the better option between:

1. replacing a highly inefficient vehicle with a slightly more efficient vehicle
2. replacing a typical vehicle with a highly efficient one.

For instance they might ask “Is it better to replace an SUV that gets 10 Miles Per Gallon (MPG) with a 15 MPG Minivan or to replace a 25 MPG sedan with a 60 MPG hybrid?” The mere asking of the question probably tells you that the answer is the less intuitive former case. Here’s why: 10 MPG is the same as 1000 gallons of gas per year for the average American car owner (10,000 miles of driving per year). The 15 MPG minivan uses 670 gallons for that same average year of driving.

This means that replacing the SUV with the minivan results in 330 gallons of saved gas. The 25 MPG sedan, on the other hand, uses 400 gallons per year whereas the 60 MPG hybrid uses 170 gallons per year, resulting in only 230 gallons of saved gas. Are you with me? The important metric here is the amount of gas saved, not the number of miles traveled on a single gallon. When viewed through the proper lens we can see increased miles per gallon leading to diminishing returns in a more important metric like gallons (or dollars) per 10,000 miles. The chart at right presents gallons of gas used when driving 10,000 miles in vehicles of various fuel economies.

So, what does this have to do with home energy efficiency? Most of us are “driving” really inefficient homes and furthermore when we try to improve our home’s efficiency we tackle the wrong things. For instance, we find ourselves focused on 10 or 20 Watts of phantom load when we could be eliminating thousands of Watts of useless air conditioning load! The SUV of your home electricity consumption is probably not your cell phone charger (if it is, you deserve a pat on the back). Focus your efforts on the big stuff like heating, air conditioning, large appliances, and jacuzzis. If you can cut the usage of the big energy hogs by a small percentage it will make a world of difference.

Update: The Duke professors mentioned in this post have launched a site with a super cool GPM calculator. Here’s the site:

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