Post-Retrofit Residential Assessments
This study examined a range of factors influencing energy consumption in households that had participated in residential energy-efficiency upgrades. The study was funded by a grant from the U.S. Department of Energy’s Pacific Northwest National Laboratory and was conducted by faculty and staff of Portland State University Center for Urban Studies and Department of Economics. This work was made possible through the assistance and support of the Energy Trust of Oregon (ETO), whose residential energy-efficiency programs provided the population from which the sample cases were drawn. All households in the study had participated in the ETO Home Performance with Energy Star (HPwES) program. A number of these had concurrently pursued measures through other ETO programs. Post-retrofit energy outcomes are rarely investigated on a house-by-house basis. Rather, aggregate changes are ordinarily the focus of program impact evaluations, with deviation from aggregate expectations chalked up to measurement error, the vagaries of weather and idiosyncrasies of occupants. However, understanding how homes perform post-retrofit on an individual basis can give important insights to increase energy savings at the participant and the programmatic level. Taking a more disaggregated approach, this study analyzed energy consumption data from before and after the retrofit activity and made comparisons with engineering estimates for the upgrades, to identify households that performed differently from what may have been expected based on the estimates. A statistical analysis using hierarchal linear models, which accounted for weather variations, was performed looking separately at gas and electrical use during the periods before and after upgrades took place. A more straightforward comparison of billing data for 12-month periods before and after the intervention was also performed, yielding the majority of the cases examined. The later approach allowed total energy use and costs to be assessed but did not account for weather variation. From this statistical analysis, 18 study participants were selected and interviewed. The participants completed an in-home interview covering a range of topics, including changes in occupancy and additional changes to the homes that may have affected energy use. The goal of the interviews was to identify factors that may have contributed to unusual energy performance. These factors were identified by their frequency of occurrence in outperforming or underperforming homes, or simply by identifying factors that had the largest impact on overall savings. The motivations and levels of satisfaction with the outcomes of the upgrades were covered in detail, as well as extensive discussions of behaviors pertaining to thermal control, lighting, water, and appliance use. Most of cases studied achieved substantial energy savings, although it was more common for the projected savings to be greater than the demonstrated savings. Two factors that played a very large role in savings variation were 1) changes in occupancy and 2) fenestration improvements outside of the incentive programs. Motivation for pursuing the upgrades (e.g., environmental sustainability vs. comfort or cost savings) did not seem to play any role in achieving savings. Participants generally were more concerned with maintaining aesthetics through lighting than comfort through heating or cooling. They also seemed more likely to turn the lights off when leaving a room than to turn the heat off when leaving the home.