Do Connected Thermostats Save Energy?

Connected thermostats (CTs) manage HVAC systems in over four million homes. Widely varying strategies are used by these thermostats to reduce HVAC energy use. Thermostat vendors claim savings of up to 20%; however, there is no accepted procedure to evaluate the effectiveness of these strategies. Presently, consumers (and utilities) have no way to identify the most effective CT products. We developed a method to quantify HVAC energy savings from a CT and assign a savings metric to CT products based on the method. The method collects indoor temperature and HVAC run time data from thermostats, plus publicly available local weather data. Temperature data is then regressed against HVAC run time to develop a unique HVACthermal model for each home. CT savings are expressed as percentage HVAC run time reduction from that with an assumed constant temperature baseline. To assign a metric value to a product (hardware plus service), savings from a large number of homes using the product are aggregated via a specific procedure. The method is being tested on large groups of thermostats from several vendors. Many of the strengths and weaknesses of this approach have been identified and will be discussed, along with anticipated future improvement of the method.