Heating degree days and cooling degree days

Degree days are a way to measure how warm or cold a region is. When it comes to energy, knowing about the heating degree days or cooling degree days your location experiences can help you understand how much energy you might need to use for home climate systems. In this article, we take a look at both cooling degree days and heating degree days and their impact on your energy use, plus potential ways to shift your behavior based on the degree days in your region.

One way to roughly quantify how much energy you'll need to heat or cool your home is with degree days. Specifically, a cooling-degree day is a measure of how hot the temperature is during a period of time, and a heating-degree day is a measure of how cold the temperature is during a period of time. Both cooling degree days and heating degree days are calculated by comparing the difference between the average outdoor temperature and a standard temperature, typically 65°F. Using this calculation method takes into account how extreme the outside temperature is - more extreme temperatures lead to more heating or cooling degree days.

To calculate cooling degree days for a region, subtract 65°F from the average temperature of a day. For example, if a summer day has an average temperature of 85°F, that day has 20 cooling-degree days (85°F - 65°F = 20). The higher the cooling degree days, the hotter the region is.

For heating degree days, the calculation is the opposite: subtract the average temperature of the day from 65°F. For example, if a winter day has an average temperature of 30°F, that day has 35 heating degree days (65°F - 30°F = 35). The greater the heating degree days, the colder the region is.

Degree days based on location

To understand how much energy you'll need to run air conditioning and heating systems where you live, you can add both the cooling degree days for the whole year and heating degree days for the whole year. The Energy Information Administration (EIA) provides these f calculations for each census region in the U.S., as seen in the below graphics.

Cooling degree days by region

Heating degree days by region

New England has the most heating degree days per year, while the West South Central census region experiences the most cooling degree days. These numbers mean that you will use more energy for heating in New England compared to any other region and more energy for air conditioning in the West South Central when compared to the rest of the country.

Degree days provide a mechanism to assess how much energy a certain location needs for climate control systems (heat and air conditioning). A home in the northeast that experiences a high number of heating degree days will need to power a home heating system much more often throughout the year than a home in a warmer location, like the southwestern region of the U.S. However, that southwestern home will have a much greater need for air conditioning due to the relatively higher number of cooling degree days.

Importantly, heating and cooling degree days are tied to solar energy in two main ways: potential impacts on panel efficiency and opportunities for energy bill savings.

1. Solar panel performance depends on temperature, in part

An important metric in determining how much power a solar panel produces is its temperature coefficient. Solar panel temperature coefficient is a measure of how much less efficient a solar panel gets for every degree the outside temperature is above about 77°F (25°C). Solar panels perform better in cold temperatures, and solar panels installed in regions with warmer weather will perform slightly less efficiently than those installed in cooler climate areas.

Therefore, you can look at the annual degree day numbers in your area to get an idea of how well a solar panel installation will generate electricity. Regions with relatively high cooling degree days have higher average temperatures, and solar panels can be expected to perform less efficiently in these areas.

Regions with more cooling degree days are likely located further south and receive more sunlight hours throughout the year, so the annual impact of increased temperatures may be offset partially or entirely by the abundance of sun. Even though the panels themselves will be less efficient at converting that sunlight to electricity in warmer regions than in colder regions, a solar panel system will likely still produce more energy throughout the course of the year in warmer climates. To get an idea of your solar energy potential, check our our Solar Calculator, which takes into account your geographic location when estimating how much you might benefit from installing solar panels.

2. More degree days = more energy usage you can offset by installing solar

Degree days are a handy way to quickly understand how much you are spending on energy relative to other regions. If you live somewhere with a high total number of heating and cooling degree days, you can save significantly on your energy bills using free, renewable energy from a solar installation on your property if your home climate control systems run on electricity.

Solar pairs well with air conditioning, but heating systems are not always run on electricity. Fortunately, there are several electric heating technologies that are either already common or are emerging options that transition your home's heating systems away from fossil fuels like gas and oil. Electric radiators, air source heat pumps (ASHP), ground source heat pumps (like geothermal systems), electric furnaces, and electric boilers are all viable home heating options that you can power with a solar energy system.

Don't have an electric heating system? It may be worth your money and time to retrofit your home with an electric system powered by solar panels for several reasons. From a financial standpoint, going with electric heating and installing solar panels insulates you from the highly variable costs of fossil fuels. With solar, you pay more upfront, but once you reach your solar payback period, you can generate free electricity right on your property for the lifetime of your solar panels, often more than  30+ years. Additionally, there are positive environmental impacts to consider. Going with an electric heating system plus solar reduces your carbon footprint and instead shifts your energy consumption to clean, renewable solar energy.

Regions with higher levels of heating and cooling degree days may see some of the highest savings from installing a solar energy system but installing solar panels to offset your electricity use is a financially viable option just about everywhere in this country. The best way to start your solar shopping process is to compare quotes from solar energy installers side by side, and the best way to compare is with the EnergySage Solar Marketplace.

Our network of qualified, pre-vetted installers will provide you with no-obligation quotes on our platform for you to look at side by side, so you can make the best energy choice for your property. If you are interested in electrifying your home climate systems, simply leave a note on your property profile. Installers can often recommend other local companies specializing in heating and cooling systems or even do the installation themselves in some cases.