Have you ever noticed your energy use goes up one month and down the next, even when your routine (or thermostat) hasn’t changed? Outdoor temperature is often the cause.
Utilities use something called weather degree days to measure how much heating or cooling is needed during a given period. Degree days compare the average outdoor temperature to 65°F, which is considered a standard indoor comfort baseline.
There are two types of degree days:
Cooling Degree Days (CDD) measure the amount of cooling required when temperatures rise above 65°F.
If the average temperature is 80°F, that day equals 15 cooling degree days (80 – 65 = 15).
In hotter months, higher cooling degree days usually mean:
– Air conditioners run longer
– Fans and dehumidifiers run more
– Refrigerators work harder
– Homes gain heat faster, especially with sun exposure or poor ventilation
Heating Degree Days (HDD) measure how much heating is needed when temperatures fall below 65°F.
If the average temperature is 45°F, that day equals 20 heating degree days (65 – 45 = 20).
In colder months, higher heating degree days usually mean:
– Furnaces run longer
– Space heaters get used more
– Water heaters may work harder
– Homes lose heat faster, especially when insulation levels are low
Even if you keep your thermostat at the same setting, your heating system may have to run more often during a colder-than-normal stretch.
WHY THIS MATTERS FOR YOUR BILL
The weather has a big impact on how much electricity you use, and what you see on your energy bill. The more degree days, the harder heating or cooling systems have to work to keep your home comfortable. When those systems run more often, energy use typically goes up as well.
TIPS TO REDUCE WEATHER-DRIVEN ENERGY USE
While we can’t control outdoor temperatures, small efficiency steps (sealing air leaks, maintaining HVAC systems, and adjusting thermostats) can help manage weather-related energy use.
Understanding degree days can help you better interpret seasonal changes in your energy bill and make informed decisions about home energy use.