Comparing Your House
to Others

 

Kouba-Cavallo Associates Homepage

  Do you know how well your house is doing in terms of the energy it uses? Is your house operating efficiently or inefficiently? If you knew that your house was among the most energy-efficient homes around, then you could brag to your friends that the house is saving you money, not wasting scarce energy resources, and not unnecessarily polluting the air.

But what if you discovered that your home uses much more energy than similarly sized houses in your area? Of course, if your house is an architectural shrine to Mies van der Rohe, with floor-to-ceiling glass walls separated by heat-conducting steel beams, you may adore the house so much that you don't care that the structure is acting as a giant outdoor radiator. There may be little you want to do about the large utility bills that you are paying.

However, if you don't adore your inefficient home that much, you may want to talk to friends or to a professional home performance contractor about how to improve your home's energy use so that you can cut your utility costs. Until you know how your home's energy use ranks, though, it's difficult to decide if you need to worry about making your home perform better.

Crude Benchmarking
How do you figure out where your home ranks in terms of its energy use? You can compare your energy use to other similarly sized homes in your area, using a process known as benchmarking. For most homes-but not for the unique architectural masterpiece--the admittedly crude benchmarking method described here will be adequate to give you a good idea of your home's efficiency ranking. The method relies on you computing the fuel used to heat your house during the past year and normalizing that fuel use for the size of your home and last year's weather. Fuel use is measured in BTUs (British Thermal Units) rather than gallons or therms. Nevertheless, the conversion is easy since there are 95,475 BTUs in each gallon of propane and 100,000 BTUs in each therm of natural gas. (Electric space heat needs to be evaluated differently because space heating usage must be separated from electricity usage for air conditioning and appliances. If your house has electric space heating consider using the Home Energy Savers audit from Lawrence Berkeley National Laboratory.)

What's a BTU?

One BTU (British Thermal Unit) is the amount of heat needed to raise the temperature of one pound of water one degree Fahrenheit. It is approximately the heat energy released by striking a kitchen match!

Start by totaling up the number of gallons of propane or therms of gas that you used last year and multiply by the appropriate conversion factor (95,475 or 100,000) depending on which fuel you used. You will find the amount of fuel used on your utility bill. For instance, if your home used 900 gallons of propane during the past year, you used 85,927,500 BTUs of energy.

Next you need to calculate the square feet of heated space that is in your home. Many houses are not simple rectangles, but they can be thought of as being made up of rectangles. You can sketch your house as if each floor consisted of a series of connected rectangles and measure the length and width of each rectangle. The square feet of floor space of each rectangle is the product of its length and width. Add up the square feet in each of the rectangles and you get the total floor space of your home. For instance, suppose you have an L-shaped 1-story home as in the illustration below. The left portion of the house is a rectangle of 40 feet in length and 25 feet in width. The square footage of this rectangle is 40 x 25, or 1,000 square feet. The right portion of the house is 28 feet in length by 30 feet in width, or 840 square feet. The total floor space of the two rectangles is 1,840 square feet.

   
Finally, you need to know the weather during the past year, because how cold it was outside will affect how much heat you need to use to keep a house comfortable. For space heating purposes, weather is often measured in Heating Degree Days (HDD), which indicates how much colder the temperature is outside compared to a reasonably comfortable temperature of 65 degrees Fahrenheit (F). A day in which the average temperature is 35 degrees-F is said to have 30 HDD because the average temperature is 30 degrees-F below the 65 degrees-F baseline. A day with an average temperature of 25 degrees-F is said to have 40 HDD. The Heating Degree Days for each day of the month or year are added together to give a measure of how cold the month or year was. The average number of Heating Degree Days in the Chicago area is around 6,536 HDD each year. In the year 2000, however, there were 6,242 HDD as measured at Midway airport. Often the Heating Degree Days are given on your monthly utility bill - so you just need to sum them up for the year. If the Heating Degree Days are not given on your utility bill, you can find this information at the National Climatic Data Center's web site http://www.ncdc.noaa.gov. The National Climatic Data Center is part of the U.S. Department of Commerce.

Now let's put this all together. Suppose your 1,840 square foot building used 900 gallons of propane last year, and that the winter weather conditions resulted in 6,400 Heating Degree Days. You can get a pretty good idea of how your home compares to similar buildings by multiplying 900 by 95,475, dividing your answer by 1,840, and dividing that answer again by 6,400. The result is 7.30 BTUs per Square Foot per HDD.

So is this good or bad?????

You can compare the result to the table below to determine how your home is doing relative to other single family homes in cool and cold portions of the U.S. (the states of the Northeast, Midwest, and West). The table has been developed from a U.S. Department of Energy survey that was taken in 1997. It shows that slightly less than one-eighth of all single family homes using propane or natural gas in the Northeast, Midwest, and West used less than 5 BTUs per SqFt per HDD; and about 10 percent used more than 25 BTUs per SqFt per HDD. If your house uses 7.30 BTUs per SqFt per HDD, you are not among the best or the worst. Almost certainly there are things you could do to improve the way your house uses fuel and to save money.

Table: Distribution of Single Family Homes in the Northeast, Midwest, and West Census Regions using Propane or Natural Gas as the Primary Space Heating Fuel in 1997

 Range of Energy Use

Realtive Frequency
 Under 5.0 BTUs/HDD/Sq.Ft.

12.2 percent
 Between 5.0 and 10.0 BTUs/HDD/Sq.Ft.

39.4 percent
 Between 10.0 and 15.0 BTUs/HDD/Sq.Ft.

23.9 percent
 Between 15.0 and 25.0 BTUs/HDD/Sq.Ft.

14.7 percent
 Over 25.0 BTUs/HDD/Sq.Ft.

9.8 percent

Improved, But More Complex
The method described above is only one approach to benchmarking. A better, but more complex, method separates out the fuel used for space heating from that used for baseload purposes, like water heating. Andy Padian wrote a fine article that describes a such a method in the November/December 1999 issue of Home Energy magazine. (He looked using fuel oil for space heating, a fuel popular in the Northeast.) If your home is using more than 15 BTUs per SqFt per HDD, you should certainly investigate whether your house is wasting energy through baseload or space heating.

An Internet-based tool for analyzing your home is available from the U.S. Environmental Protection Agency and Lawrence Berkeley National Laboratory. This tool, called the Home Energy Saver, allows you to input information about your home, including the square feet that we mention above. You can analyze your home and determine its energy conservation opportunities. The web site for this tool is found at http://hes.lbl.gov/.

Also if you are considering any kind of renovations of your home, you should look into ways to save energy. A reasonable standard of energy efficiency is 5 BTUs per SqFt per HDD for space heating and 75,000 BTUs per unit per day for water heating and other baseload uses. You can meet or beat these standards without breaking the bank and more than pay back the incremental renovation costs with the savings from lower energy bills. To learn about these methods, consult almost any issue of Home Energy magazine, or contact a home performance contractor.

 

For additional information, contact James Cavallo, Ph.D., Principal, at 630 971-2016 or send e-mail to cavallo\@Kouba-Cavallo.com.

This page was last updated on January 14, 2008.