Getting to Net Zero on a Budget

Abstract

Only a few homes in the US have demonstrated net zero energy performance over a full year. Of the successful net zero energy projects for detached single-family homes, most have been newly constructed, rather than retrofits. A cost-effective retrofit strategy for single-family homes could expand net zero options to a broader segment of the housing stock, a segment with a relatively high energy consumption per household. Also, most net zero energy projects are limited to the structures on the property, and are focused on energy, rather than greenhouse gas (GHG), primarily carbon dioxide (CO2) emissions. Expanding the scope to incorporate not only energy from structures, but also a household's CO2 emissions from structures and vehicles would enable the evaluation of strategies to reduce all Scope 1 and 2 emissions. These are the emissions that a household has direct control over, namely the direct combustion or emission of CO2 and the emissions caused by the household's use of electricity.One family's net zero project in Washington, DC, aims to demonstrate cost-effective attainment of annual zero net CO2 emissions not only from the retrofitted home itself, but from the household's two vehicles. In this way, the household will lower its net Scope 1 and 2 emissions to zero. The project's house-related energy reduction investments to date consist of a ground source heat pump, solar hot water, spray foam insulation, lighting upgrades, new windows, programmable thermostats, and other energy efficiency retrofits. These investments and activities have resulted in 65% reduced energy consumption and 32% reduction in house-related CO2 emissions (before taking into account on-site solar generation). Over the course of a year, the total house-related energy consumption of 33 million British thermal units (MMBtu) (mostly electricity with some natural gas for cooking) was lower than the 36 MMBtu electricity generated by the 9.8 kW of roof-mounted solar photovoltaic (PV) panels.To reduce the household's net Scope 1 and 2 GHG emissions to zero, vehicle-related CO2 emissions also were addressed in the project. Excess electricity generation from the PV panels is used to offset electricity required to power an electric vehicle for daily commuting requirements (up to 8,000 miles annually). A second vehicle is primarily fueled by biodiesel and used for longer trips.By August 2014, four years after the project began, the project had demonstrated that for certain markets, commercially available energy technologies can enable a suburban single-family detached house to be cost-effectively retrofitted and operated to achieve net positive GHG emissions. The household achieved a year of net zero house-related electricity, energy, and emissions in October, 2013, possibly becoming the first home to achieve that distinction in the District of Columbia. For the 12 months to September, 2014, combined net building and vehicle (Scope 1 and 2) CO2 emissions of 1.9 MT was 86% less than annual building and vehicle emissions prior to the project of approximately 13.6 MT CO2. The household is expected to have demonstrated 12 months of net zero emissions for the home and two vehicles (Scope 1 and 2) in 2015.The project also demonstrates a process for determining a combination of energy saving technologies with potential to reduce emissions sharply, possibly up to net zero overall household emissions, at a relatively low cost and impact to occupants.The family that carried out the project funded the project at its own expense, without any sponsorship or support other than government incentives available to the general public. Total energy related investments after state and federal incentives are approximately $85,000. The family expect to see a complete return on these energy investments in 10 years.