What is cooling degree days and energy consumption and how to use heating degree days to calculate energy consumption?
Degree days are a simplified form of historical weather data. They are commonly used in energy monitoring and targeting to model the relationship between energy consumption and outside air temperature.
Weather normalization of energy consumption is one of the most common such uses of degree days. In theory, weather normalization (or "weather correction") enables a like-for-like comparison of energy consumption from different periods, or from different places with different weather conditions.
At the risk of stating the obvious, weather normalization is only appropriate for energy consumption that is affected by the weather, which mainly means building heating and cooling, and refrigeration.
Degree days are the difference between the daily temperature mean, (high temperature plus low temperature divided by two) and 65°F. If the temperature mean is above 65°F, we subtract 65 from the mean and the result is Cooling Degree Days. Cooling degree days (CDDs) are a measure of how hot the temperature was on a given day or during a period of days. A day with a mean temperature of 80°F has 15 CDDs. If the next day has a mean temperature of 83°F, it has 18 CDDs. If the high temperature for the day was 92°F and the low was 68°F, the mean temperature for the day would be 92 + 68 = 160 / 2 = 80°F. The Cooling Degree Days would then be 80 - 65 = 15 CDD. The simplest way to normalize energy-consumption is to calculate the kWh per degree day for each kWh energy-consumption figure in question. Simply divide each kWh figure by the number of degree days in the period over which that energy was used. Heating and cooling degree days (HDD and CDD) essentially indicate how hot (or cold) it is outside for a given day and for how long it was at that temperature. This can be more helpful than knowing the temperature alone for estimating how much energy you used on heating and air conditioning. A first estimate of an annual heating energy consumption of our house in watt-hours would be the heat loss coefficient, 127.3 W K–1, multiplied by the number of degree days multiplied by 24 (to convert from days to hours). Dividing by 1000 then gives the result in kilowatt-hours (kWh). If the temperature mean is above 65°F, we subtract 65 from the mean and the result is Cooling Degree Days. If the temperature mean is below 65°F, we subtract the mean from 65 and the result is Heating Degree Days. Because the result is below 65°F: 65°F - 29°F = 36 Heating Degree Days. Heating degree days are a measure of how much (in degrees), and for how long (in days), the outside air temperature was below a certain level. They are commonly used in calculations relating to the energy consumption required to heat buildings.