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Programmable thermostats help save money by resetting the temperature when homeowners are asleep or away. But setting them up can be painstaking, and 89 percent of users never get them out of manual mode. The Nest thermostat requires almost no setup and teaches itself when to adjust the temperature.

It starts by building a schedule. For the first week, users change the temperature normally. The Nest notes their preferred at-home temperature, say 72°F, and also determines appropriate “away” temperatures—60º in winter and 80º in summer, for example. To account for conditions outside, the Nest checks the weather over Wi-Fi, and its indoor humidity sensor tells it when to kick in the fan for comfort. If everyone leaves, a motion sensor signals the processor to activate the away setting.

Even small tweaks can save cash. A change of a single degree from the preset program can reduce power consumption by 2 to 5 percent. The Nest also has a ZigBee wireless chip, so it can work with smart meters to turn on the A/C or the furnace when energy is cheapest.