Have you wondered why your energy bill could be different each month, or year to year, even if you are doing everything the same? The answer is probably the weather.
Weather is one of the biggest factors that affects how much energy you use, especially during temperature extremes.
Hot summer weather means your air conditioner works harder to maintain a constant comfortable temperature in your home. When that changes to frigid winter weather, your furnace runs around the clock to keep your house warm. In both cases your energy bills are higher because of increased energy use from heating or cooling.
Makes sense! Except, how do you know just how much the weather is influencing your bill over a given time and not caused by other factors? Or how would you learn how much money a new energy-efficient appliance is saving you if the weather and usage is always changing?
That’s where degree days come in. Degree days create a standard to account for the changing temperature when looking at energy use. On your bill, you can find the average temperature, heating degree days and cooling degree days.
Degree days are based on differences of the average temperature for each day from a baseline of 65 degrees. One degree day is equal to the average temperature for that day being 66 or 64 degrees. The more extreme the average daily temperature is from 65 means a higher number of degree days for that day. If the average daily temperature is 85 degrees, there are 20 degree days for that day alone. The number of degree days for each day are added up and shown on your bill for the month.
In the cold winter months, when your furnace runs, there are heating degree days. The colder it is, the more heating degree days there are for that day. In the warm summer months, when your AC is running, there are cooling degree days. Months when the weather is changing, like May and September, can have both heating and cooling degree days.
For example, if you experience a December with 250 heating degree days in one year and 300 heating degree days in December of the next year, with all other things being equal, you’d expect a 20% increase in your bill from weather-related usage alone. Just be sure you’re comparing similar months, like February to February or August to August.
Looking at degree days helps you understand energy use over time, no matter how the weather changes. As you make behavioral changes and energy improvements at your home, remember that comparing energy use alone might not reflect savings.
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