Calculating Energy Usage Per Heating Degree Day

If you have a high heating bill, is it because your home is an energy hog? To get a true measure of your home's energy efficiency, you must measure (1) energy usage and (2) the average temperatures for the same period. To do this accurately, look up historical heating degree days for your zip code.

What is a Heating Degree Day

Heating Degree Days (HDD) is a measure of the difference between the outside temperature and a base temperature (usually 65 degrees F) for a given time period. For example, let's say on Monday the average temperature was 50 degrees, then the HDD for the day is 55 (65-50=15). On Tuesday, it was colder with an average temperature of 35 and the HDD for the day is 30 (65-53=30). Because Monday had 15 HDD and Tuesday had 30 HDD, Tuesday will have roughly double the energy consumption for heating. Degreedays.net provides HDD data for zip codes all over the country.

The Goal - Energy Loss Per HDD

To determine the energy efficiency of the home, the goal is to determine how much energy a home loses for each HDD. In other words, how much energy does your home have to use to keep it at a certain temperature for each HDD.

Tracking Energy Usage

Every day or every few days, record your meter reading or readings. If you have natural gas heating, the meter will track therm usage. Your electric meter will measure KWH used. When I did my experiment, I tried to remember to record the meter reading each morning around the same time.

Electric Usage Counts

Even if you have natural gas for home heating, you still should track electric usage. Most electric usage contributes to heating the home. Your light bulbs, computers, toasters, coffee makers, dishwashers, etc. all convert most of the electricity they consume into heat. I don't feel as guilty in the winter for leaving the lights on, because they simply take some load of the gas furnace. (Of course, electricity is more expensive.)

Daily Calculation

To illustrate how the calculation works, here are two examples of data from two days with different temperatures.
Day 1:Mild, HDD:10 Therms:2 KWH:12 Therms:2 Day 2:Cold HDD:30 Therms:6 KHW:15 Gas Energy Used = 6 Therms * 10K BTUs/Therm * 0.8 furnance efficiency = 60K BTU KWH Energy = 15 * 3400 BTUs/KWH = 2K BTU 62KBTU/ If you take a bunch of readings, you can get a more accurate calculation.

Hot Water Adjustment

Since we also have a gas powered water heater, it is not correct to measure 100% of the gas usage going to heat. I made a rough estimate that we lose about a half a therm a day to heating hot water that does not contribute to the heating of the house.

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