Calculations for Energy Use in Driving

Jan 12, 2012 | Autos

These calculations have been prepared for GHSP Inc., in support of the PBS series

EARTH: THE OPERATORS' MANUAL

Martha Amram
October 2011

The goal of these calculations is to create a very simple, yet reliable estimate of fuel use per household for driving. Energy used for driving is the largest single energy using activity in most households.

CALCULATIONS

We are a nation of drivers. Recent data shows that 59% of households have two or more cars. [1] Note that in the interactive web tool, the user is prompted to enter data for each vehicle owned, so no default is used.

A reasonable estimate of the average number of miles driven per vehicle is 12,000.[2]  Note that in the interactive web tool, 12,000 miles per year is provided as a smart default and the user can override this value for each vehicle.

The third input to the calculations is the miles per gallon (mpg) of each vehicle entered into the web form.  The interactive tool provides an opportunity to look up the mpg by make, model and year of the vehicle manufacture. The data are from the DOE.[3]  The user is also offered the opportunity to override the input.

For each car the energy used from driving is calculated as:

BTU per year per car =

Miles per year / Miles per gallon X BTU per gallon

The energy content of a gallon of gasoline is 124,238 BTUs.[4] There are 20.42 pounds of CO2e per gallon of gasoline.[5]

In sum, our estimate of energy used in driving is the product of smart defaults and user updates.

OTHER CONSIDERATIONS

In a recent paper on urban density and vehicle miles driven per year, Erwing and Cevero (2010) survey a number of studies that find that 60 - 90% of the variation in miles driven per year across individuals is due to the density of their residential area.[6]  Further, the authors find that across many studies this relationship is inelastic, that it has not changed much in response to changes in exogenous conditions or other non-density factors in urban areas.  One implication of this research is that it may be ineffective to ask residents to drive less to save energy.

An alternate energy saving requests could be to work from home one day a week, or to take public transit one day per week.  These actions could save 20% of the weekday driving energy  -  and there are few behavior changes that can save so much energy in a single action.

As noted in the goods and services section of this white paper, more than 85% of the lifecycle energy used to make and use a car comes from driving. So given that residents may not be able to drive less due to the driving distances to work, schools and downtown areas, choosing a higher mpg car could be an important energy saving action. Particularly when high mpg cars are not always the highest priced cars, better consumer understanding of the lifecycle costs of driving could sway purchase decisions.

TIPS FOR SAVING ENERGY

· BUY A HIGH MPG CAR.  90% of the lifetime energy used by driving is the gasoline.  Only 10% of the lifetime energy goes into car manufacture.  So, when buying a car, push yourself to get the highest miles-per-gallon (mpg) car that fits your needs. You'll save energy and money.

· CONSOLIDATE TRIPS.  More than half of our driving is for trips 6 miles or less.  Consolidate those many small trips and achieve substantial savings.

· WORK AT HOME ONE DAY PER WEEK. This is an easy way to cut your driving to work by 20%.

NOTES

[1] http://nhts.ornl.gov/tables09/fatcat/2009/household_HHVEHCNT.html
[2] EPA reviews the various data sources and draws this conclusion. See http://www.epa.gov/otaq/climate/documents/420f11041.pdf
[3] The EPA data tables were downloaded from this website: http://www.fueleconomy.gov/feg/download.shtml
[4] http://www.eia.gov/kids/energy.cfm?page=about_energy_conversion_calculator-basics#mogascalc
[5] http://www.epa.gov/otaq/climate/documents/420f11041.pdf
[6] Erwing and Cevero (2010), "Travel and the Built Environment", Journal of the American Planning Association, downloaded from here: http://www.tandfonline.com/doi/pdf/10.1080/01944361003766766

Keywords:  emissions from driving; car emissions; energy use by autos; auto energy use; emissions per gallon.