What Are Degree-Days?


… And How Do They Affect My Utility Bills?

You may have a gut ‘feeling’ that this year was warmer than last year, and expect lower heating bills as a result – but how do you know for sure? Enter the degree-day, a measure of how much heating (or cooling) you have to do to keep your house comfortable.

Let’s say you’re comfortable at 68°F, and let’s also say the outside weather is only 66°F. This means your furnace needs to provide an additional 2°F. If the outside temperature remains the same for the entire day, your furnace will have to provide 2 degrees for the entire day to keep you comfortable – or 2 degree-days. If you’re keeping score in Celsius, your comfort level is 20°C, and the outside temperature is 19°C all day, so your furnace has to provide 1 degree-day of additional heat to keep you happy. As long as you’re consistent in keeping track of degree days (Fahrenheit or Celsius), you can keep accurate records of your energy requirements.

You don’t need to record temperatures all day though – just head over to BizEE Degree Days (a site maintained by Weather Underground), put your location in the blank next to “Weather Station ID”, and click on “Station Search”.

Finding a nearby weather station

As shown below, Weather Underground will then show you a number of weather stations near your location, some maintained by individuals, some by commercial interests, and some maintained by airports. Pick a location near you; note that Weather Underground suggests you picking on an airport weather station if possible, as their data is typically more accurate. Once you have clicked on the station in the list provided, the station’s ID will appear next to the Weather station ID as shown below. Great – just a few more steps!

Specifying the weather station

Next, click on the type of degree-day you’re interested in. Are you interested in the number of heating-degree days, or the number of cooling-degree days? You may actually be interested in both, if the temperature drops significantly at night and becomes hot during the day. If that’s the case, you just need to run the tool twice. Similarly, pick whether you use Fahrenheit or Celsius to track your usage.

The “Base temperature” is a little trickier. You’re indicating how warm you like the house while heating, or how cool you like it if you use an air conditioner, but you need to keep in mind that there are other sources of heat in the house – people, lights, appliances, etc. For example, the outside temperature may be 63°F, while your house hovers at 68°F because someone took a shower, someone else was doing laundry, the lights were on, the sun was throwing a lot of heat in through several south facing windows, etc. If this is a typical day (and if you are comfortable at 68°F), your base heating temperature is 63°F. With the base heating temperature set at 63°F, your house will be 68°F, you’ll be comfortable – and your furnace won’t contribute any heat at all.

Conversely, if you use less lighting on some summer day, if you have deciduous trees shading those south facing windows, and assuming you’re less likely to bake cookies and pies during the heat of the summer, your house may hover at 80°F while the outside temperature is 76°F.  If your comfort zone is about 74°F, you should indicate your base cooling temperature is 70°F. This lets Weather Underground know that your air conditioner will have to cool your house an additional 4°F to keep the inside temperature at 74°F.

By setting the base temperature as indicated above, you’ll get a more accurate picture of the energy needed to heat or cool your house on any given day. Recognize that you can always put on a sweater during cool nights or chat with a neighbor under a shady tree on a hot afternoon – but for many, the house regulates it’s temperature automatically. The base heating and cooling temperatures help you understand the process better, and improve the accuracy of the data returned by Weather Underground.

Finally, choose how often you’d like to have the data reported (once a month, once each day, etc), and select how much historical information you’d like to have. That’s it! Click on “Generate Degree Days” and wait just a moment for Weather Underground to send your heating or cooling degree day information back to you. It’ll arrive in the form of a spreadsheet which you can view with a program such as Open Office or Microsoft Excel.

Next up – what do I do with all this information?

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