Methodology for Estimating Winter Energy Savings

The following explanation pertains to electricity, which was selected as an example. Similar equations would be used for the four other energy sources (natural gas, fuel oil, kerosene, and LPG).

Data from the 1997 RECS:
(variable names in all capitals)

• BTUELSPH: the estimate of the amount of electricity (in Btu) consumed for space heating for each respondent.

• HDD65: the heating degree-days to base 65 degrees Fahrenheit (° F) for the respondent's location.

• HDD60: the heating degree-days to base 60° F for the respondent's location.

• HDD55: the heating degree-days to base 55° F for the respondent's location.

• HDD50: the heating degree-days to base 50° F for the respondent's location.

• Heating Degree-Days (HDD): A measure of how cold a location is over a period of time, relative to a base temperature. (In RECS reports, the base temperature used is 65° F and the period of time is 1 year.) The number of heating degree-days for a single day is the difference between the base temperature and the day's average temperature, if the daily average is less than the base; it is zero if the daily average temperature is greater than or equal to the base temperature. The number of heating degree-days for a longer period of time is the sum of the daily heating degree-days for days in that period. Average daily temperature is the mean of the maximum and minimum temperatures for a 24-hour period. Heating degree-days can also be calculated using a base temperature other than 65° F. That computation is performed in an analogous manner.

Major assumptions:

1. All households have thermostats or can adjust the setting of their space-heating equipment in a way that has the same effect as lowering the thermostat by 1° F.

2. The thermostat setting has an effect only on the consumption of the fuel that is the main space-heating fuel.

3. The effect of lowering the thermostat by 5° F is equivalent to raising the outdoor temperature by 5° F.

4. We use the difference between HDD65 and HDD60 to estimate the change in heating degree-days if the outdoor temperature were 5° F warmer all year long.

5. The effect of dropping the thermostat by 1° F is one-fifth of the effect of dropping it by 5° F.

6. The heating degree-days for a year spanning the the 2000-2001 heating season (July 2000-June 2001) will be the same as for the 1997 calendar year.

Equations:

1. (HDD65 – HDD60)/5 approximates the change in degree-days (base 65) if the outdoor temperature is 1° F warmer all year long.

2. We use 100 × {[(HDD65 – HDD60)/5] × HDD65} to estimate the percent drop in space-heating consumption as a result of lowering the thermostat by 1° F.

3. We use BTUELSPH × {[(HDD65 – HDD60)/5] × HDD65} to estimate the drop in electricity consumption for space heating.

Changes in assumptions that would alter the estimates:

If HDD60 > 0 then

(HDD65 – HDD64) > (HDD64 – HDD63),
(HDD64 – HDD63) > (HDD63 – HDD62),
(HDD63 – HDD62) > (HDD62 – HDD61), and
(HDD62 – HDD61) > (HDD61 – HDD60).

Hence, {[HDD65 – HDD64] × HDD65} is larger than {[(HDD65 – HDD60)/5] × HDD65}. Consequently, the drop in consumption as a result of lowering the thermostat by 1° F is more than one-fifth the drop as a result of lowering the thermostat by 5° F.

Note:

If {[(HDD60 – HDD55)/5] ×HDD60} is larger than {[(HDD65 – HDD60)/5] × HDD65} then
{[(HDD55 – HDD50)/5] × HDD55} is even larger.

Therefore, using {[(HDD60 – HDD55)/5] × HDD60} or {[(HDD55 – HDD50)/5] × HDD55} to estimate the effect of a 1° F drop in thermostat setting will give a larger estimated change. Since (HDD65 – HDD60) is larger than (HDD60 – HDD55), a drop in the thermostat by 10° F will have less than twice the effect as a drop by 5° F.

Applying Estimates to this Winter

Many readers may want to use these data to determine the fuel and cost savings they could realize this winter by lowering thermostat settings. The values in this article represent a rough guide at best. The data in this article are based on known 1997 weather conditions and the fuel use that resulted. To the extent that the weather this heating season is different than it was during the 1997 heating season, fuel savings and cost savings would be affected. Also, savings are calculated as averages over a large population of homes. To the extent that fuel use varies in individual homes (due to their size, amount of insulation, or other characteristics), potential fuel and cost savings will also vary from home to home.