Update February 2012 : You can now get degree-days for anywhere on the planet calculated for free by kWIQly
Given a stream of dry-bulb temperatures (normal Celsius or Fahrenheit ignoring wind-chill) - We can write down how we calculate heating degree-days for any particular balance-temperature really easily:
For each regular temperature measurement in a day write down 0 if
temperature is above balance-temperature. Otherwise write down the difference between balance-temperature and air-temperature. Degree days is the average of all the numbers written down.
Here is a really simple worked example....
|Time||Air Temperature||Degree-Load for 15.5 Balance|
So here we see a day with 5.71 "Degree-Days" of load - It is simply the area in "degree-days" under the horizontal balance temperature line - Shown here in Red - The lower the Temperature (Green) - The bigger the red area relative to the balance or "on-set of heating" line