Friday, October 05, 2007

Our Electricity Use Over the Years

In order to assess both how the solar panels and the geothermal heating and cooling have impacted our energy usage, we need to understand our electricity use. The first step in doing so is finding out how much we have used historically. The recent past data can be obtained from the power company. Connecticut Light & Power keeps the last 2 years' data online for easy access, but we have more.

While digging through old boxes full of bills looking for heating oil invoices, I also collected the electricity bills and cross-checked them with old check registers. As a result, a history of our usage from the time we moved into our house until present has been compiled, and it is given in the first figure.

The red line shows, according to Connecticut Light and Power, how much energy we pulled from the grid, and the blue line shows how much we have used. The difference is the amount we generated from our solar panels since May, 2006. The amount generated doesn't look like much, and compared to what we now use, it isn't, but the details deserve its own discussion, and now is not the time.

There is a lot of structure in this plot that needs to be explained. The big drop in usage from late 2006 to May, 2006 is the renovation phase. We moved out of the house in this period, and the amount shown is what was used during the reconstruction.

Again, we have to divide the house into two periods: (1) the old house (2000-2005), and the newly renovated house (May, 2006 to present). Let's look at the old house first.

Interestingly, our electric usage peaks in the winter and not in the summer as the national and regional power does. The reasons for this are many, namely we didn't have central air, we had one room that was fully electric heat, and we used a couple of small space heaters in some of our draftier rooms. Although the summer usage was lower, a small summer peak is present in the data, and it got bigger from 2001 to 2005, because over that time we purchased more window air conditioner units. 2005 was also a particularly hot summer (next figure).

The big spike in our electricity use starts in November, 2006 with the turning on of the geothermal heat pumps. Clearly our electric use has gone up significantly, but as the last post showed, our heating oil demand decreased significantly. So which one has won out? The newer system is more efficient, but by how much, we still have to estimate. I'll get into the details in the next post, but before that, this part cannot end without showing the energy use versus the average temperature, and that is given in the next figure.

There are four main groups in this plot. The lower purple circles are during the construction phase and can thus be ignored. The blue dots are prior to the renovation, and the red dots are after. The four red dots falling below and to the right of the others are from the brief period where we had a new house, but the central air wasn't in, for we were awaiting the completion of the geothermal system. We had our four window air conditioners going full blast during this period, and interestingly enough, the data match up pretty well with the old house in the 2005 period (the dots hotter than 60 degrees closest to the lower four red dots). The house was not at all comfortable in this period, and needless to say, the final completion of the geothermal was welcome indeed, even though it came too late for the 2006 cooling season.

There is a lot of scatter in the blue data, and there are a couple of reasons for that. The first is that electricity demand is a combination of heating and lighting, and it is when the days are shorter that they are generally also colder. Part of the demand is for lighting, and de-tangling the two effects may not be possible. Other contributing factors are guests and vacations, and these effects have not been taken out. A final factor adding to the scatter is that in the earlier data, some of the data are estimates and not actual readings. This was prior to the time that CL&P had electronic meters, and reading the meter was much more variable. I've noticed that the newer readings are all "ACTUAL" readings and not estimates. On the warmer side, as previously noted, the amount of cooling capacity increased over time, and that and vacations explain a good deal of the scatter during the warmer periods.

Finally, I just want to say that while January, 2005 represented our peak heating oil use, January, 2007 represents the high water mark for us in our electric usage, or at least I believe it will be. We are working on reducing that peak value, and hopefully the new changes will be in before the winter comes...and the changes aren't the switching to fluorescent light bulbs, which, I'm somewhat embarrassed to say, we still haven't done as yet.


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