Energy Products & Services
Understanding Heating, Cooling Degree-Days
Knowing about heating and cooling degree days can help homeowners
and businesses alike understand why some months their energy bills
are 5%, 10%, or even 20% higher or lower. The total cost to heat
a home or business is primarily determined by outside temperatures.
The temperature difference between the inside and outside of a
building is the driving force that determines heat loss for the
structure. As outside temperatures drop, heat is conducted through
walls, ceiling, and the floor at a faster rate. Other factors,
like air infiltration (leakage of air) also play a role, but most
of a building's heat loss is from conduction through the building's
exterior surfaces.
In order to quantify the relationship between outside temperature
and an affect on building heat loss, scientists came up with the
concept of degree-days. It is based on
two assumptions:
- At an outside temperature of 65 degrees F, no heating or cooling will be needed in the building.
- The difference between the mean outside temperature and 65 degrees
F, is directly proportional to the energy use of a building
for heating or cooling.
Therefore, knowing how far the outside temperature got from 65 degrees
F on each day of the month, tells us how much of a heating month
it was, compared to the long-term average. Note that the mean
temperature in a 24-hour period is the mid-point between the
two extremes of daily high and low. It does not take into account
the duration at these temperatures. Even so, the mean temperature
is a reasonable indicator to use of daily conditions.
How do I calculate the heating degree-days for a particular day?
Find the day's average temperature by adding the day's high and
low temperatures and dvide by two. If the number is above 65,
there are no heating degree-days that day. If the number is
less than 65, subtract it from 65 to find the number of heating
degree-days.
Example:
If the day's high temperature is 60 and the low is 40, the average
temperature is 50 degrees. 65 minus 50 is 15 heating degree-days.
Therefore, the temperatures on this day resulted in a total of
15 heating degree-days. It is a measurement of the heating requirement.
Summing the daily figures gives a monthly total; this is the
figure that helps you make useful comparisons. Ultimately, you
want to determine how the total degree-days for a given month
compare to long-term historical averages.
How do I calculate cooling degree-days?
Cooling degree-days are based on the day's average minus 65.
They relate to the day's temperature to the energy demands
of air conditioning.
Example:
If the day's high is 90 and day's low is 70, the day's average
is 80. 80 minus 65 is 15 cooling degree-days. Summing the daily
totals gives a monthly total; this is the total that helps you
make useful comparisons.
Heating and cooling degree-days can be used to relate how much more or
less you might spend on heating or air conditioning if you
move from one part of the country to another. Of course you'd
have to take into account how well insulated your new home
will be in comparison to your old one and the different costs
of electricity, gas or heating oil. You could also use records
of past heating and cooling degree-days to see if the money
you've spent on insulation, or a new furnace or air conditioner
is paying off. To do this, you'd also need records of past
energy use.
To get up-to-date heating or cooling degree-days, click
here.
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