MPG for Homes

May 06, 2009
May/June 2009
A version of this article appears in the May/June 2009 issue of Home Energy Magazine.
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Energy consumption is by far the most important criterion in green home design and performance. Limiting energy use far outranks such factors as choosing materials and considering energy sources in determining the greenness of a house.

Energy conservation can be achieved in two ways: by reducing the amount of primary energy consumed to supply the useful-energy requirement (energy efficiency), and by reducing the end point use of nonessential energy. Energy efficiency in homes can be increased by improving the thermal performance of the building and by installing high-efficiency heating-and-cooling equipment, lighting, and appliances.

Unfortunately, incorporating energy efficiency measures does not automatically reduce energy use. Although the efficiency of residential building shells, space-conditioning equipment, lamps, and appliances has improved significantly in recent years, total residential energy consumption in the United States is increasing—and this is projected to continue. Reasons include the increased size of homes and changes in the number, type, and uses of electrical and electronic devices.

Therefore, it is essential to recognize the importance of the second way to achieve energy conservation in homes—by reducing the use of nonessential energy. This requires a raised consciousness, education, and awareness on the part of occupants, as well as a willingness to put their knowledge into practice. There are numerous and well-publicized practices that occupants can adopt to reduce their consumption of nonessential energy. These include adjusting thermostats (lower in the winter and higher in the summer); turning off lamps in unoccupied rooms; lowering the temperature setting on the water heater; turning off electrical equipment completely when it is not in use; and space conditioning only occupied rooms. How effectively these measures reduce overall energy consumption depends on the behavior of the occupants.

It follows that the actual energy performance of homes is affected by both fixed and variable factors. Fixed factors include design parameters, space conditioning equipment, lighting, appliances, and so on. The variable factor is occupant behavior. This concept also applies to the energy performance of passenger vehicles, which depends both on the characteristics of the vehicle itself and on the behavior of the driver. There exists a simple, easily calculated, and universal energy performance indicator for passenger vehicles in the form of the fuel utilization measurement, which is calculated in units of miles per gallon (mpg) or kilometers per liter. Consumers have access to published fuel utilization ratings of vehicles and can calculate actual values for their own vehicle-driver combinations. Unfortunately, a well-known, commonly used, and analogous energy performance indicator does not exist for home-occupant combinations.

How can homeowners and others measure, compare, and evaluate the actual energy performance of home-occupant combinations? I advocate three simple and easily calculated indicators. The first characterizes the thermal energy performance, the second characterizes the electrical energy performance, and the third characterizes the total energy performance of a home.

Thermal Energy Performance

Space conditioning is typically the largest single component of energy consumption in homes. The performance of home thermal envelopes and envelope-heating system-occupant combinations during the heating season can be estimated, measured, compared, and evaluated by calculating the home heating index (HHI). The HHI is defined as the annual heating load divided by the amount of conditioned-space floor area divided by the number of heating degree-days (HDD). The unit is Btu/ft2/HDD (kJ/m2/HDD). The HHI is particularly helpful because it makes it possible to compare the thermal performance of thermal envelopes and envelope-heating system-occupant combinations in different climate regions. Table 1 rates the thermal performance of homes based on the HHI.

The HHI of the thermal envelope of a house (designated as the envelope or shell HHI) can be estimated via calculations based on envelope design parameters, such as R-values and air leakage rates. The HHI of envelope-heating system combinations can be estimated via calculations based on both the envelope design and heating system design parameters, such as combustion and distribution efficiencies.

The actual HHI of an occupied house is a simple and straightforward calculation once the amount of energy used for heating is determined, based on billing records. If combustion fuels, such as natural gas, propane, or fuel oil, are used for space conditioning but not water heating, the amount of fuel used for heating is directly available from billing records. This amount of fuel is multiplied by the amount of energy per fuel unit to determine the total energy consumed per heating season.

If combustion fuels are also used for water heating, one must estimate the amount of fuel consumed for this purpose. One way to do this is to base water-heating energy consumption on the amount of fuel used in the shoulder months during the spring and autumn seasons, when little or no supplemental heating is required. This monthly amount of fuel is then subtracted from the total amount billed for each of the heating months to determine the net amount of fuel actually required to heat the home. A sample worksheet for calculating the actual HHI for these cases is shown in “Home Heating Index (HHI) Sample Calculation Worksheet.”

The actual HHI can also be calculated when wood is used as a major or supplemental source of heat. However, estimating the amount of energy in wood fuel is more difficult, due to variability in the moisture content of wood. When the wood fuel is measured on a volume basis (by the cord), estimating the amount of energy is especially difficult due to the density of the wood and the volume of the free air space compared to the volume of the actual wood.

The envelope HHI for all-electric homes and the actual HHI for such homes with electric-resistance heat can be calculated using the procedures described above. However, values obtained for the actual HHI for all-electric homes with heat pumps must be interpreted on a different scale, due to the effect of the coefficient of performance (COP) of heat pumps on energy use.

Although a home cooling index similar to the HHI has not been established, the combination of factors that improve thermal performance in terms of heating (lowering the HHI) will also improve thermal performance in terms of air conditioning. Therefore, the HHI can function as an indirect measurement for a home cooling index. Also, the air conditioning load has a major impact on residential electrical energy consumption. This impact is discussed in the following section.

Electrical Energy Performance

The residential housing sector is a major consumer of electrical energy. The amount of electrical energy used in homes varies widely, depending on many factors. These factors include the energy source for space and water heating; the climate (particularly when the climate calls for air conditioning); the type of space conditioning equipment; the size of the house; and the number and behavior of the occupants. One way to account for some of this variability and still have a useful indicator is to measure the actual electrical energy performance of home-occupant combinations using the home electrical energy index (HEEI). The HEEI is defined as the annual electrical energy consumption divided by the area of conditioned space. The unit is kWh/ft2 (MJ/m2).

Calculating the actual HEEI is simple and straightforward once the amount of electrical energy used annually is determined, based on billing records. Once the HEEI is known, comparisons and evaluations can be made on a regional basis for the following three common space- and water-heating combinations: (1) combustion fuel for both, (2) electrical energy for both, and (3) combustion fuel for space heating and electrical energy for water heating.

Unfortunately, there are almost no published data on residential electrical energy consumption (including HEEI values) that are useful for this purpose. One source provides typical values for two categories of homes—natural gas space heating and all-electric—for various regions of the United States; see Table 2. The effect of air conditioning on electrical consumption is clearly evident in this table.

Total Energy Performance

Residential housing accounts for about 20% of the total energy consumption in the United States. A useful indicator for measuring the actual total energy performance of home-occupant combinations is the home energy index (HEI). The HEI is defined as the total annual energy consumption divided by the area of conditioned space. The unit is Btu/ft2 or kWh/ft2 (MJ/m2). As with electrical energy, the amount of total energy used in homes varies widely, and for much the same reasons. In fact, for all-electric homes the two indexes are the same.

Calculating the actual HEI is simple and straightforward once the amount of combustion fuel energy and/or electrical energy used annually is determined, based on billing records. Once the HEI is known, comparisons and evaluations can be made on a regional basis for various energy source combinations. Representative values for the HEI are shown in Table 3.

MPG for Houses

I would like to see simple, easily understood, and standard energy performance indicators (such as HHI, HEEI, and HEI) become the norm for home-occupant combinations, just as the fuel utilization measurement has become the norm for passenger vehicle-driver combinations. The calculation and use of such indicators as the HHI, HEEI, and HEI, combined with the plethora of information available in print and on the Web, can go a long way toward helping homeowners to reduce energy consumption and live greener.

I will share a personal example. My wife, Susan Choma, and I live in a simple, straightforward, and economical energy-efficient home located in north central Ohio, which we designed and built in 2000. The all-electric home is well-insulated, tight, and properly ventilated and has baseboard resistant heat and a well pump (see “Design, Construction, and Performance in Ohio” HE Jan/Feb ’07, p. 36 for detailed information about the house). We practice sensible and well informed—but not drastic—energy conservation measures. The average values for HHI and HEEI/HEI for the home to date are 1.4 Btu/ft2/HDD (28 kJ/m2/HDD) and 5.4 kWh/ft2 (206 MJ/m2), respectively. These results for the three energy performance indicators clearly document in a simple and easily interpreted format the actual energy conservation achievable in a practical, real-world situation.  

Allen Zimmerman is a professor of engineering technology and technical physics at the Ohio State University, Wooster Campus.

>> For more information:
Krigger, John, and Chris Dorsi. Residential Energy. Helena, MT: Saturn Resource Management, 2004.
Wilson, Alex. Your Green Home. Gabriola Island, BC: New Society Publishers, 2006.
Scheckel, Paul. The Home Energy Diet. Gabriola Island, BC: New Society Publishers, 2005.
Amann, Jennifer, Alex Wilson, and Katie Ackerly. Consumer Guide to Home Energy Savings 9E. Gabriola Island, BC: New Society Publishers, 2007.
Johnston, David, and Scott Gibson. Green from the Ground Up: Sustainable, Healthy, and Energy-Efficient Home Construction. Newtown, CT: The Taunton Press, 2008.

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