About a year ago I bought a Nest programmable thermostat with the expectation it would eventually save me money.  Since I already had baseline data with my original “dumb” thermostat, it took me about a year to get good comparison data with the new thermostat.  As I have historically done I recorded the electricity used each day and matched it up to the average temperature recorded by the weather service at the closest airport.  This was then used to derive a formula for daily energy usage versus average daily temperature.  These curves are primarily a function of the size of my house, the efficiency of my heating system, my house’s insulation, and the setpoint of the thermostat  (The thermostat is typically set at 70°F with the new thermostat allowing the temperature to be set back in the winter  to 65°F at night and when no one is home.)  As expected when it’s colder outside it takes more energy to heat the house.  The same thing happens in the summer, the warmer it is outside the more energy it takes to cool the house. 

 

The next chart shows the average monthly cost for my location based on the above curves.  It also includes assumptions for the average monthly temperature at my location and the cost of electricity at my location.  January is the highest cost month with its low winter temperature.  May and September are the lowest since they are between heating and cooling season.  July has a small increase due to air conditioning costs.  Notice how insignificant summer cooling costs are versus winter heating costs at my location!

 

So how did my thermostat turn out?  The difference between the blue and red lines is $77 per year.  Therefore it’s going to take over 3 years for the new Nest thermostat to pay for itself.