Truth and Consequences of Home Energy Ratings

In spite of the widespread interest in HERS, there has been startlingly little documentation to answer this question. Without this information, we risk building an elaborate financial structure upon a shaky technical foundation--a sure prescription for failure.

In this issue of Home Energy, we offer one of the first published analyses to address the question of HERS accuracy (see "Home Energy Rating Systems: Actual Usage May Vary"). As you might expect, there is some good news and some bad news. The good news is that HERS tools appear to predict energy use reasonably well for new homes in extreme climates. New homes are easier to model, partly because auditors can more accurately estimate insulation levels, furnace efficiency, and other energy-related features. Even these results, however, depend upon scanty data. Nevertheless, we are confident that HERS will work for these homes, especially as the next generation of HERS tools reaches the field.

The bad news covers several fronts. The admittedly sparse data suggest that ratings of existing homes, and especially those in mild climates, are not very accurate. We aren't convinced that the sources of error can ever be completely fixed. Other approaches, such as use of utility bill histories, may be more suitable for existing homes.

Even if the HERS tools can accurately estimate energy use, the ratings will be misleading. A gas-heated home will almost always receive the same rating as a similar electrically heated home, even though the gas-heated home's utility bills will be lower--an outcome of the political influence of electric utilities. We maintain that the best home energy rating is the estimated annual utility bill.

There is no way for energy ratings to reflect the variability in human behavior. At best, a HERS tool will successfully predict energy use for a group of homes, even though individual homes may be far from the prediction. In addition, most energy ratings cover only heating, cooling, and water heating. In many homes, these end uses account for less than half of the utility bill. This creates a potential public perception problem when occupants discover that their actual energy bills are nowhere near those predicted by the HERS tool. Consumer education is critical here.

Home energy rating systems are a good idea, and they deserve support. But they also need to be carefully validated.

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