Smart thermostats, what they do and what they save

A smart thermostat is a Wi-Fi thermostat that adjusts the heating and cooling settings on its own. Heating and cooling take almost half of the average household's energy bill, more than $900 a year. The savings come from cutting how long the heating and cooling equipment runs.

What it does

• A schedule it learns. Many learn your temperature preferences and set a schedule that moves to energy-saving temperatures while you sleep or are away.
• Geofencing. It can tell when you've left and set the system back, then tell when you're heading home and bring the temperature back up.
• Phone control. You can change the temperature from anywhere with an internet connection.
• Use data. It reports how the equipment runs and the temperatures it holds.
• Updates. It may update its software with newer energy-saving features.

What it saves

A certified smart thermostat saves about 8 percent of heating and cooling bills on average, or about $50 a year. Homes with high heating and cooling bills can save up to $100 a year. To earn the ENERGY STAR label, a model has to show savings from a year of data in sample homes across the country, checked by a third-party certification body.

What moves the number

Climate, your comfort settings, how often the house is empty and the heating and cooling equipment all change the savings.

• Big seasonal swings. Homes with extreme temperature changes over the year generally save more.
• Mild climates. These usually spend less on heating and cooling, so savings run slightly lower.
• Someone always home. A home that's always occupied saves less, whatever the climate.
• Variable-speed equipment. A variable capacity heat pump or air conditioner generally works best with the thermostat its maker recommends, certified or not.

Compatibility and connection

A smart thermostat has to be compatible with your heating and cooling system. The very highest efficiency equipment, such as an air conditioner rated 20 SEER or higher, may need a controller from the same company. The thermostat should be reasonably close to the Wi-Fi router so it doesn't drop the connection, and a range extender helps when it's too far. Certified models can work with utility demand response programs.

A new thermostat is wired into the heating and cooling system, so the install is a job for an HVAC technician or an electrician.

An annotated example

What to do next

Write down the make and model of the furnace, heat pump or air conditioner, and check how far the thermostat is from the router. Thermostat settings that cut the bill covers the schedule itself, smart or not.