What is the Home Energy Index?

Can one determine a home’s energy efficiency by asking a seller for evidence of past energy performance, such as gas or electric bills? Hardly, because a home owner can chose to heat his home with a wood-buring fireplace, space heater, or some other temporary heating solution to simulate a more energy-efficent home, or decide to spend the winter in Floriday and set the heat to 55° in the meantime.

Another possible approach for determining an energy efficiency score for residential homes would be to audit the building and its heating and cooling systems using a score card, by awaring points for certain facts that are known to enhance energy-efficiency, and by substracting points for others facts that are known to cause higher energy consumption. Examples for the former are the presence of certain types of insulation, a home’s form factor, the energy rating of the furnace, etc. Examples of the latter would be the lack of certain types of insulation, etc. This sounds like a sound approach, because home owners cannot easily fudge such an audit. The problem is that any such approach does not reflect the actual performance of a home, because many contributing factors cannot be audited by just looking at a property. For instance, a home that is on the north face of a hill, or behind some big trees, gets less direct sunlight in the winter and will be more expensive to heat than a home on the sunny side of that same hill. Insulation can be present, but be poorly installed and have hidden faults that are not readily visible to the auditor, yet can have a big negative impact on energy performance.

A standardized score indicating a home’s true energy efficiency can only be determined by measuring the performance. Such measurements must be made over time to eliminate statistical or situational inaccuraries, and must include 3 components:

• the external temperature
• the inside temperature
• the activity of the heating system (i.e. capturing on-off cycles)

With these three measurements taken over at least a 7 day period during the heating season, the performance of a home can be calculated from the temperature gradient, the ratio of heat-on vs. off times, and the speed at which the home cooles off when the heat is shut off. This technique has a number of advantages:

• if a home owner reduces the set temperature on the thermostat, the temperature gradient will be smaller
• if a home site on a lot that has favorable (higher) ambient temperatures, the calculated temperature gradient will be smaller as well
• the sizing of the heating system resulting in shorter on-cycles), the