Houses Versus Cars
When trimming your energy budget, don’t forget transportation
You’re striving to minimize your carbon footprintAmount of carbon dioxide and other greenhouse gases that a person, community, industry, or other entity contributes to the atmosphere through energy use, transportation, and other means. , and your house is energy efficient. Through diligent conservation efforts, you’ve greatly reduced the amount of natural gas and electricity required to run your home.
Bravo! But how does your residential energy budget compare to your transportation energy budget? You may be happy to brag about your low utility bills — but if you’re anything like me, you’re probably a little ashamed of your gasoline budget.
How much energy are you pumping into your car? To figure out the relative importance of residential energy versus vehicle energy, let’s do the math.
According to data compiled by the Energy Information Administration, the average American household spent $1,841 on residential energy in 2005, not including transportation. Consumption of energy for space heating, hot water, and electrical appliances averaged 97,734,040 Btu (site energy) per household:
Of course, residents of cold states use more fuel for space heating than residents of warm states. Different areas of the country tend to use different fuels; while residents of rural Maine use more heating oil, kerosene, and firewood than the national average, they use much less natural gas.
If you want to perform your own calculations, here’s how to convert fuel use to Btu:
What about gasoline?
The best data I found on automobile transportation are from 2001. In that year, the average American household had 1.9 vehicles; these cars averaged 23.4 miles per gallon. On average, each household bought 908 gallons of gasoline (108,960,000 Btu) to drive 21,252 vehicle miles.
In other words, we use about 11% more energy for personal automobile transportation than we use to run our homes. If you’ve managed to cut your residential energy use to a level that is significantly below the national average, then transportation is probably responsible for the bulk of your carbon footprint.
You can estimate the actual number of gallons of gas you use each year if you know how many miles you drive and the mileage of your car. The number of gallons used to drive 21,252 vehicle miles per year would, of course, be less if your car gets more miles per gallon than the national average:
- If your car gets 25 mpg, you use 850 gallons per year.
- If your car gets 30 mpg, you use 708 gallons per year.
- If your car gets 35 mpg, you use 607 gallons per year.
Those who drive fewer miles use less gasoline. When counting miles driven, remember to include all of the vehicles in your household. A gallon of gasoline contains about 120,000 Btu.
The average American flies 1,055 miles per year. Assuming 60 passenger miles per gallon of jet fuel, the average American burns 17.6 gallons of jet fuel (2,110,000 Btu) per year. Like kerosene, jet fuel contains about 130,000 Btu per gallon.
Since the average number of people per American household (according to 2005 Energy Information Administration data) is 2.57, the average household energy budget for flying is 5,422,700 Btu per year.
If the members of your household fly more than 1,055 miles per year, your aviation energy budget will, of course, be higher.
So, to summarize:
- Residential energy use: 97,734,040 Btu per household per year.
- Automobile energy use: 108,960,000 Btu per household per year.
- Air travel energy use: 5,422,700 Btu per household per year.
The total energy budget for these three line items is 212,116,740 Btu. The percentages for each line item:
- Residential energy use, 46%.
- Automobile energy use, 51%.
- Air travel energy use, 3%.
So get an efficient car already!
Sharp-eyed readers may quibble with some of my numbers — noting, for instance, that the cited transportation data correspond to a different year than the residential data. Of course, energy use numbers and energy prices vary from year to year, and vary regionally. But these broad averages point to important facts:
- Transportation energy use is significant. The average American family uses more energy to operate their cars than their home.
- Many of us are willing to spend hundreds of dollars on a new water heater, in hopes of saving only 10% on our water heating bill. But we could save twice as much energy by simply choosing a car that gets only 1 mpg better mileage than the average car.
- Anyone who succeeds in living without a car should be honored. Good work, bicycle commuters! Keep pedaling.
Last week’s blog: “Resisting the Allure of Small Wind Turbines.”
- Tom Burke
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