The great North American cold wave of 2014 – also known as the Polar Vortex – has got us all wishing winter would just get done, but thanks to Mr. Groundhog, it doesn’t look like that is happening any time soon. With these record low temperatures, we are also experiencing a record number of high bill inquiries. Here is a little information to keep in mind on how the cold weather may be affecting your bills.
Quantity Compares Heating to Normal Temps
In fact, the single greatest factor affecting monthly heating bills for residential customers is outside temperature. This temperature difference between inside and outside is the “driving force” that determines heat loss for a building. There is a value that power suppliers use to describe the temperature conditions that occur during the billing periods. It is called degree days. Having a quantity that characterizes the month’s local weather conditions (heating requirements) is invaluable.
To quantify the relationship between outside temperature and building heat loss, the concept of degree-days was developed decades ago. Most weather stations provide records of degree-days, and monthly summaries, which can be accessed on-line. The concept of degree-days is based on two assumptions:
1. At an outside temperature of 65°F, little or no heating or cooling will be needed within a building.
2. The difference between 65°F and the mean outside temperature is directly proportional to the energy use of a building for heating or cooling. The mean temperature for the day is the mid-point between the two extremes of daily high and low.
Therefore, knowing how far the outside temperature got from 65°F on each day of the month provides an indicator of heating (or cooling) needs. Note that the mean temperature is just the mid-point between the daily high and low. Even so, this temperature is a reasonable indicator of the day’s conditions, and the requirements for heating or cooling.
For Example: On a day when the high temperature was 42°F and the low was 20°F, the number of Heating Degree-Days (HDD) is found as follows:
1st: Calculate the Mean Temperature: (42 + 20)/2 = 31°F
2nd: Subtract this Mean temp. value from 65 to get degree-days: 65-31=34 HDD
3rd: Calculate each day; then add together to get an accumulated monthly total.
Fortunately, weather stations throughout the country calculate degree-days, and list them free on-line. Every state has multiple recording sites that offer data, but they can be hard to find if you don’t know where to look. Reviewing a comparison of the degree days on your current bill to last month, or even last year can help you understand why your bill is fluctuating.