In 2004, a consortium of entities active in the conservation of energy in the Pacific Northwest, including the Bonneville Power Administration (BPA), the Northwest Energy Efficiency Alliance (Alliance), the Energy Trust of Oregon (Trust), the Northwest Power and Conservation Council (NPCC), Idaho Power and other regional utilities funded an indepth study of heat pump performance in the Pacific Northwest. Heat pumps have enjoyed a significant increase in popularity in recent years, both with the public and with utility program designers. This study used a variety of analytical methods to assess the overall performance of heat pumps in Northwest climates and to identify the factors that have the most impact on the efficiency achieved. The study design was intended to address a number of project goals: