Heat Pumps & Outside Weather

How a heat pump works and why outside temperatures affect your bill

It’s cold outside! This means your heating and air conditioner works harder and uses more electricity to keep you warm and comfortable. Let us explain why your energy use rises when the temperature outside goes down.

Your heating and air conditioner — called an HVAC system — uses forty to sixty percent of your monthly electricity. This means your electric bill can go up depending on how hot or cold the outside temperate was.

The Middle Tennessee average annual climate ranges between thirty to ninety degrees, which is why the majority of homes in Middle Tennessee use electric heat pumps as their HVAC system. 

Using thermodynamic principles, heat pumps use outside air to heat and cool your home. In the simplest terms, when you want heat, your heat pump pulls cold air out of your home and uses the warmer air outside to heat it back up again. This method allows your heat pump to warm your home a few degrees at a time. It’s why they are one of the most cost-effective and efficient ways to heat or cool your home.

But, when outside temperatures drop below forty degrees, your heat pump cannot warm your home fast enough to make up the difference between the outside temperature and your thermostat setting, so it uses a secondary heat source called auxiliary heat. 

You can think of auxiliary heat like a giant space heater. It consumes three times more electricity when it uses this feature compared to normal. However, it also generates direct heat that you can feel. This is the same feature that kicks on when you raise your thermostat more than two degrees at a time. The more electricity the auxiliary heat uses, the more money you’ll pay on your bill. With this basic understanding of heat pumps, let's look at how the weather impacts energy usage.

We'll use the usage graphs in myMTEMC app to illustrate what’s happening. You can view your energy consumption in daily, weekly, and monthly intervals using the myMTEMC mobile app or by logging into your account on MTEMC.com

This graph shows readings for an all-electric home, along with the weather data. If you use other heat sources like natural gas, you’ll see a similar increased energy usage pattern when the temperature drops. Natural gas and other heat source HVAC systems still use electric motors and fans to push air into your home.

The thermostat in this home was set to 68 degrees. On this day, the low temperature was 30 degrees and used 88 kWh. The following day, the temperature outside dropped to 28 degrees and used 109 kWh. Because the weather dropped below forty, we know that the auxiliary heat worked to keep the home at 68 degrees. You can tell by the spikes on the graph when your auxiliary heat turned on and off. As the temperature drops, those spikes increase in frequency or duration.

The longer the temperature outside stays below forty degrees, the longer your auxiliary heat has to run and the higher your energy consumption is at the end of the month. Unfortunately, this means your bill will increase.

We hope this helps explain why cold weather may cause you to see a higher-than-normal bill.

If you get a high bill, we recommend you check your energy consumption to find ways to change your habits to reduce how much electricity you use. 

Did you know you can save almost three percent on heating costs for every degree you drop your thermostat? You can learn more ways to save energy on our social media pages or by going to MTEMC.com/Tips.

There’s one last thing I’d like to remind you about regarding high bill season. Your bill may have more or fewer days in your billing cycle. For instance, January has thirty-one days compared to February’s twenty-eight. This can make your bill seem high or low depending on your billing cycle. More days in a month mean more energy used that month!  You can find the number of days your bill covers by looking at a copy of your paper bill.

If you have any questions about your bill, your account, or are looking for more information to save electricity, we hope you give us a call at 877-777-9020. Have a great day!

Energy Service Coordinators

Rutherford & Cannon
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David Willhite


Murfreesboro City

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Katie McKee



Mark Kendrick


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Phillip Price


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