Evan Mills
Lawrence Berkeley National Laboratory
MS 90-4000, Berkeley, CA 94708
Lawrence Berkeley National Laboratory Report No. 50950
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(Version subsequently published in Energy & Buildings available here)
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There exist hundreds of web- and disk-based building energy software tools. Exhibiting considerable range in approach and creativity, some are highly specialized while others consider the building as a whole. Users are faced with a dizzying array of choices and, often, conflicting results. We evaluated 50 web-based residential calculators and 15 disk-based tools. Some require a relatively small number of well-considered inputs while others ask a myriad of questions and still miss key issues. Thus, more detail does not automatically translate into a “better” or “more accurate” tool. Efforts to quantify and compare the "accuracy" of these tools are difficult at best, and prior tool-comparison studies have not undertaken this in a meaningful way. Accuracy evaluations are inherently limited by the availability of measured data. Furthermore, certain tool outputs can only be measured against “actual” values that are themselves calculated, while adequate measured data is rarely available. Many factors conspire to confound performance comparisons among tools, and the sources or implications of observed differences in results are difficult to identify. For the tools we tested, predicted energy bills for a single test building ranged widely (by nearly a factor of three), and far more so at the end-use level. Most tools over-predicted energy bills and all over-predicted site energy consumption. The over-predictions ranged as high as $1400 per year (approximately 250% of the actual bills). Energy savings estimates automatically generated by the tools varied from $46/year (5% of predicted use) to $625/year (52% of predicted use). We also discovered a remarkable number of results that suggest errors in programming or algorithm accuracy. There are numerous potential avenues for improving residential energy tools. Synthesizing the information gathered, we developed best-practice guidelines that may be useful to developers of residential-energy tools. More coordinated funding and planning of tool development could help address the fragmentation of development and deployment efforts that has hampered tool quality and market penetration thus far.