Where heat pumps end up costing money!

I have a dual stage heap pump that services my home and for the most part it does a great job. That is, as long as the outside temps don’t get much below 35°F that is…

The graph above represents actual data of high and low temperatures taken November 2012. For this far in the month, the temperatures have average spot on to historical averages for the last thirty years; about 47°F. So, in fact, this has been a pretty average month, so far. (The mean temperature standard was taken from the weather service's figures for West Plains Mo. for the month of November and rounded).

Now, take a look at the area I’ve marked in yellow – there were 10 out of the 20 days where the overnight lows got down below 35°F. I took these days and averaged out the kilowatt hours of power that was used for those days and came up with 26 kWh. Next, I took and averaged out the power consumed for the other 10 days where it didn’t get quite so cold and arrived at a figure of 18 kWh! Now, since the heap pump will tend to switch over to resistive heating elements below 35°F, I’m guessing that the difference in cost to me will be about 30% more in terms of the electric bill.I'm also guessing that, for a large part, it doesn't matter as much how warm it is during the daytime as it does how cold it gets at night. It'll be interesting to see what happens during the days when the thermometer never gets above freezing, should we experience that later on in the year.

Over the coming winter, I’ll keep an eye on this and see if it holds true.

Notes: During this period of time, the heat pump thermostat was set at 60°F.