How Can Degree Days Affect Your Electricity Usage?

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Heating degree days and cooling degree days are concepts and calculations that will have you whipping out a piece of scratch paper. But, they also play a vital role in calculating the amount of electricity homes and businesses use.

Simply put, heating degree days (HDDs) and cooling degree days (CDDs) measure the gap between hourly outdoor temperatures and the targeted indoor temperature.

For example, on a chilly winter day:

  • Time of day: 10-11 am
  • Outdoor temperature: 40 degrees
  • Thermostat setting: 70 degrees
  • Difference: 30 degrees
  • HDD/CDD hourly total: 1.25 (30 degree difference ÷ 24 hours in a day)

If the outdoor temperature and your thermostat setting stay the same for all 24 hours during that day, your HDD total would be 30 (1.25 X 24). A high HDD total like this one means your system is working hard to keep you comfortable.

In another scenario, on a mild spring day:

  • Time of day: 10-11 a.m.
  • Outdoor temperature: 70 degrees
  • Thermostat setting: 70 degrees
  • Difference: 0 degrees
  • HDD/CDD hourly total: 0 (0 degree difference ÷ 24 hours in a day)

In this example, your heater or A/C doesn’t need to run as much in order to regulate the indoor temperature. An HDD or CDD total close to zero means your system isn’t working hard to keep you comfortable.

It’s not always an apples to apples comparison to just look at electricity usage compared to last month or last year. Daily HDDs in February 2015 based on the outdoor temperature at Dallas/Ft. Worth International Airport ranged from 8.4 on the 15th to 43.7 on the 27th (assuming a thermostat setting of 70 degrees). Drastic weather swings like this by the hour and day can mean your system is working more  which means higher electrical usage to  keep you comfortable indoors.

Late February in North Texas this year was cold, but by March 3 the temperature climbed to almost 70 degrees. It’s hard to remember those cold weeks when you read your electricity bill while wearing shorts and a t-shirt.

Avoid surprises in your bill:

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EE Martin
EE Martin



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