Saturday, August 15, 2009

What are "degree days"?

I am sorry to say that this is important to saving energy. The reason I'm sorry is because it's so darn complex.

The reason we care about "degree days" is because our home's climate control system dominates our energy use. Everything else is secondary. So to properly evaluate our heating and cooling energy use we need to somehow establish a way to compare one day to the next, or week, or month or year. If we spend a bunch of money on conservation how do I really know I have done some good and quantify it.

Degree days was developed just for that purpose and is used by commercial building operators to compare their building's energy use. Before we move on let me make sure I have established the need.

Today, my air conditioning energy use was largely dictated by the weather, minute by minute. Just using the high temperature of the day doesn't really define the amount we use. It's the temperature ALL day that counts. Was it a hot night? Did it warm up fast? Did it cool down slow? All of these contribute to how hot a day it really was. In fact, one could look at the temperature every second of an entire day.

Degree days essentially takes the temperatures from the entire day, usually in half hour increments, and calculates the number of "units" accumulated through the day and that's the degree days. A "unit" would be a degree for a half hour which is 1/48th of a day. But of course its not that simple.

If you keep your house at 75 degrees on a 75 degree day you would have zero (nada) degree days. Why? Because the calculation is based on how much YOU would have used based on your thermostat. If you set your thermostat at 72, then the house would have to be cooled 3 degrees on that 75 degree day thus accumulating 3 degree days of cooling.

Bottom Line #1 -With degree days you have an accurate amount to determine the demand for energy for heating and cooling. This is very useful to compare July 2008 with 2009. For example...

The average high temp in June 08 was 96 degrees, and 94 in 09. So June was cooler this year right? Well actually, there were 284 cooling days based on my 75 degree thermostat setting in June 08. In 2009 June was 292. So June 09 was actually the hotter month. It made sense my June 09 bill was higher in KWH used, than last year.

But there is even another step. We can divide the cooling KWH used by the degree days and get a rough efficiency number thats useful for comparing months. Dividing the current month by the same month last year gives me a percentage improvement number.

Bottom Line #2 - When I compare the months, I can see the results of my energy saving efforts that are focused on climate control in my home. New insulation, sealing, more attic vents etc all are contributing to a higher efficiency number.

This link takes you to a handy site where you plug in your zip code and thermostat setting and it will give you your personal degree days. http://www.degreedays.net/

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