How 'Degree Days' Impact Electric Bills

by Richard Hiatt, Contributing Writer

The great North American cold wave of 2014, also known as the Polar Vortex, may be long over, but chances are you haven’t forgotten the resulting impact on your monthly electric bills.

As cold temperatures return, it may help to understand that the single greatest factor affecting monthly electric bills for residential customers is outside temperature. The temperature difference between inside and outside is the “driving force” that determines heat loss for a building.

To quantify the relationship between outside temperature and building heat loss, the concept of degree days was developed decades ago. The concept of degree days is based on two assumptions:

	At an outside temperature of 65 degrees Fahrenheit, little or no heating or cooling will be needed within a building.
	The difference between 65 degrees Fahrenheit and the mean outside temperature (mid-point between the two extremes of daily high and low temperatures) is directly proportional to the amount of energy a building uses for heating or cooling.

For example, on a day when the low temperature is 20 degrees and the high temperature reaches 42 degrees, the mean temperature would be (20+42)/2 = 31 degrees. The Heating Degree Days (HDD) for that day would be 65-31 = 34. Added together, each daily HDD to get an accumulated monthly total can help predict how much bills will increase during months with a large number of degree days.

Last January, during the month that experienced the Polar Vortex, the mean temperature for the entire month was only 26 degrees in many areas of the state. In comparison, the mean monthly temperature for the previous year was nine degrees higher — the mean temperature for January ’13 in many areas of the state was 37 degrees). This difference was directly reflected on the amount of energy Virginia’s co-ops needed to purchase to meet member demand, and also on bills those members received for the subsequent billing period.

For those on fixed incomes or who are otherwise concerned about their electric bills spiking during periods of peak consumption, talk with your co-op about enrolling in its budget or levelized billing program, which can help spread costs out during the year so that each month’s bill is approximately equal. 

Source: Rural Electricity Resource Council, www.rerc.org