‘Degree days’ help calculate energy needs


As many parts of the world deal with excessive heat this summer – including the Upstate – power demand is important to consider as more people turn to air conditioning to stay cool. Meteorologists and energy companies alike calculate how much power demand will go up or down based on the weather, using a lesser-known metric called “degree-days.”A degree-day is simply the difference between the average temperature on a given day and a baseline temperature – usually set at 65 degrees. Specifically, degree-days are used to estimate how much energy is needed to heat or cool indoor spaces based on outdoor temperatures on a given day.If that difference is positive and the average temperature is above 65 – it means it’s a “cooling” degree -day. For every degree above 65 we go – we add one cooling-degree day. If that difference is negative and the average temperature is below 65 – it’s a “heating” degree day. Looking at the average number of cooling degree-days per year in our area – gradually over the past 50 years – the average number of cooling degree -days per year has gone up significantly. In the 1960s and early 1970s, Greenville had an average of 1,500 cooling degree days per year.From 2012 to 2021 – Greenville had an average of 1,860 cooling degree-days per year. Using those figures – we can very roughly estimate that just based on the weather – not accounting for changes in urbanization, lifestyle etc. – we now require about 24% more energy than we did 50 years ago just to keep indoor spaces at the same temperatures.In Asheville – where climate records go back further in time – the trend in cooling degree-days per year is more significant. Between the 1910s to early 1920s and the 1960s to early 1970s – the average number of cooling degree-days per year went from 763 to 768. That represents an increase of less than 1%. Fast forward another fifty years and that number is now up to 1,123. That is a 46% increase from fifty years ago – twice the rate of increase in Greenville.” “We have a team of meteorologists to help us understand what mother nature has planned for us,” Ryan Mosier of Duke Energy told WYFF 4. ..We use that data along with our planners who spend their waking moments making sure that the grid is ready to deliver service to our customers when they need it.”Duke Energy also told WYFF 4 that the primary metric they consider with power demand is the change in cooling degree-days from year-to-year, ie changes from May last year to this past May. Duke Energy said that on-average – this past May was about 1° Fahrenheit warmer in the Duke Energy Carolinas territory than May last year, which translates to a 52% increase in the number of cooling degree-days in the month.Sign up for the latest heat-related advisories, Watches, and Warnings, and be sure to click the “Weather” tab on the homepage at WYFF4.com for the latest video from the WYFF 4 forecast Team.

As many parts of the world deal with excessive heat this summer – including the Upstate – power demand is important to consider as more people turn to air conditioning to stay cool.

Meteorologists and energy companies alike calculate how much power demand will go up or down based on the weather, using a lesser-known metric called “degree-days.”

A degree-day is simply the difference between the average temperature on a given day and a baseline temperature – usually set at 65 degrees. Specifically, degree-days are used to estimate how much energy is needed to heat or cool indoor spaces based on outdoor temperatures on a given day.

If that difference is positive and the average temperature is above 65 – it means it’s a “cooling” degree-day. For every degree above 65 we go – we add one cooling-degree day. If that difference is negative and the average temperature is below 65 – it’s a “heating” degree day.

Looking at the average number of cooling degree-days per year in our area – gradually over the past 50 years – the average number of cooling degree-days per year has gone up significantly. In the 1960s and early 1970s, Greenville had an average of 1,500 cooling degree days per year.

From 2012 to 2021 – Greenville had an average of 1,860 cooling degree-days per year. Using those figures – we can very roughly estimate that just based on the weather – not accounting for changes in urbanization, lifestyle etc. We now require about 24% more energy than we did 50 years ago just to keep indoor spaces at the same temperatures.

In Asheville – where climate records go back further in time – the trend in cooling degree-days per year is more significant. Between the 1910s to early 1920s and the 1960s to early 1970s – the average number of cooling degree-days per year went from 763 to 768. That represents an increase of less than 1%. Fast forward another fifty years and that number is now up to 1,123. That is a 46% increase from fifty years ago – twice the rate of increase in Greenville.

“We have a team of meteorologists to help us understand what mother nature has planned for us,” Ryan Mosier of Duke Energy told WYFF 4.

“…We use that data along with our planners who spend their waking moments making sure that the grid is ready to deliver service to our customers when they need it.”

Duke Energy also told WYFF 4 that the primary metric they consider with power demand is the change in cooling degree-days from year-to-year, ie changes from May last year to this past May. Duke Energy said that on-average – this past May was about 1° Fahrenheit warmer in the Duke Energy Carolinas territory than May last year, which translates to a 52% increase in the number of cooling degree-days in the month.

Sign up for the latest heat-related advisories, Watches, and Warnings, and be sure to click the “Weather” tab on the homepage at WYFF4.com for the latest video forecast from the WYFF 4 Weather Team.

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