As my wife and I have been planning to build our house, we’ve been doing a lot of research into energy usage. We will construct a passive-solar house, meaning that we intend to capture the energy of the sun to heat and cool the structure. This is how houses have been built for centuries. It’s only in the past fifty years that people started building houses without regard for their surroundings, failing to take the simple step of letting sunlight into their homes to provide warmth in the winter. One of the things that I wanted to figure out was how much of our existing energy usage, in our rental house, was for heating and cooling. That would help me to determine how much we might stand to save by building with proper insulation, heating with the sun, cooling with proper shading, etc.
The first thing that I did was figure out how much need that we have for heating and cooling. The measure of this is the “degree day.” That’s simply the measure of how far the temperature deviates from 65° each day. So if the temperature averages 70° in a 24-hour period, that would be calculated as five degree days. If it’s 0° for a 24-hour period, that’s 65 degree days. If the temperature sits at a daily average of 0° for a month, that’s 1,950 degree days (65 * 30). Using the degree day data provided by the NWS’ Climate Prediction Service, I was able to determine how many heating degree days (when the temperature was below 65°) and cooling degree days (when the temperature was above 65°) there had been in the previous year. That yielded a graph like such:
In this period, December had by far the most degree days, and September the fewest. Clearly heating is a far bigger need than cooling, since the overwhelming majority of degree days are from temperatures below 65°
The next thing that I did was combine a year’s worth of home energy usage — derived from our Dominion bills — with this data. In this graph, degree days are now represented by the line graph, while the bar chart represents kilowatt hours.
It’s easy to see that the two of these co-vary. No regression analysis is necessary to see that our power bill fluctuates in tandem with degree days. Our power bill is paying for heating and air conditioning; every other use of power pales in comparison.
I was able to use these figures to determine what our power bills are likely to run us in a properly-constructed house, one with a BTU/ft2/kWh rating closer to 1.0 than the whopping 9.2 that — using these numbers — I estimate for our current dwelling. By calculating how much we’d save with such an efficient envelope (remembering that mortgage payments are tax-deductible, while energy costs aren’t, but mortgage payments include interest) gives me the amount of money that we can afford to put into making our house more energy efficient without actually spending any more. Turns out that’s a whole lot of money.
Consider comparing degree-day data for your area to your last year’s energy usage. You may find, as I did, that you’re spending a lot of money for the privilege of not wearing a sweater around the house. In fact, I’m wearing flannel-lined jeans, wool socks, slippers, and a sweater as I type this. Live and learn.