A Net-Zero Energy Home Grows Up: Lessons and Puzzles from Ten Years of Data

In 2005, Habitat for Humanity of Metro Denver, with support from the National Renewable Energy Laboratory (NREL) and other partners, built one of the first homes in the United States to achieve net-zero energy (NZE) based on monitored data. A family of three moved into the house when it was completed and continues to live there. The home has been monitored continuously for the past 10 years. Although photovoltaic (PV) production has remained relatively steady, net energy performance has varied each year. The home was a net producer of energy annually in each of the first 3 years and in the ninth year, but not in years 4 through 8 and 10. Electricity use in the home increased steadily during the first 8 years. Miscellaneous electric loads (MELs) and space heating appear to be primarily responsible for the increase in energy use. The long-term results from this home have highlighted some research needs. MELs as an end-use category are becoming a larger fraction of whole-home energy use, especially in low-load homes, and we do not have a good way to curb their growth. Changes in people's behavior can make achieving NZE a difficult design problem that may force home builders to put larger PV arrays on homes where NZE is mandated for each home. Large amounts of solar generation can be challenging for electric utilities. Adding a requirement that all NZE homes include feedback for the occupants and grid-responsive capabilities may help alleviate some of the concerns of electric utilities.