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.

One thing I’ve always wanted to know is, what is the ideal speed to drive your car for maximum fuel efficiency?  Obviously aerodynamic drag is going to decrease fuel economy at higher speeds, but I always suspected there was a speed point below which other factors also reduced fuel economy. 

In the past I had tried recording long drives and then plotting instantaneous fuel economy versus vehicle speed, but I always suspected the data was being corrupted by coasting and accelerating.  The lower speeds tended to be either accelerating or coasting while faster speeds tended to be primarily steady state cruising.

To rectify this I found a stretch of road on the way home from work with minimal traffic so I could vary my speed without being a traffic hindrance.  Each day I drove the same stretch of road at different speeds using the cruise control.  I used the Torque app on my phone with a Bluetooth adapter connected to the car’s OBD port to record the fuel economy and vehicle speed.   Since the software can also use the phones GPS I was able to only use data that was recorded between specific latitude points so the same start and end points were always used.

As can be seen in the chart below my particular car appears to have the best steady speed fuel economy at about 35 mph.  I suspect some others factors are probably included in this data, with the largest suspect being ambient air conditions, probably air temperature or possibly humidity.  After I get some more data over a whole year I plan on making separate curves for different temperature ranges.