Degree Days

Degree Days

Weather Data for Energy Saving

Degree Days – An Introduction

Google Knol

This popular article was written by Martin Bromley from Degree to explain in simple terms what degree days are and what they are used for. It was originally published as a Google Knol where it received the Top Viewed Knol Award Top Viewed Knol Award and the Top Pick Knol Award Top Pick Knol Award. We republished it here when Google Knol was discontinued, and have made some small edits since then to try to make it clearer and more useful.

Degree days are a specialist kind of weather data, calculated from readings of outside air temperature. They are used extensively in calculations relating to building energy consumption. Heating degree days give an indication of the energy consumption required for heating (in cold weather); cooling degree days give an indication of the energy consumption required for cooling (in hot weather).

If you want degree days for a location near you, I suggest you head over to Degree But, if you want to understand what degree days really are (and personally I think you really should understand this before using them...), please read on.

There are three main types of degree days: heating degree days (HDD), cooling degree days (CDD), and growing degree days (GDD). I've focused most of this article on explaining heating degree days. Once you understand how heating degree days work, it will be very easy for you to understand the others.

Heating degree days

Heating degree days are a measure of how much (in degrees), and for how long (in days), the outside air temperature was below a certain level. They are commonly used in calculations relating to the energy consumption required to heat buildings.

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Why are heating degree days useful?

The energy consumption of building heating systems is more complicated than the energy consumption of TVs, kettles, or computers. You can't just plug a heating system into a Kill-A-Watt meter to find out how much energy it uses each hour, because the energy usage of a heating system varies with the weather...

Essentially, the colder the outside air temperature, the more energy it takes to heat a building.

If you live in the Caribbean, it's probably warm enough that you won't need heating at all; if you live in New York, you'll probably only need heating in the winter; if you live at the North Pole... you'll probably want your heating on all year round.

But the outside temperature doesn't just vary from one location to another – it varies all the time, wherever you happen to be. It's usually colder at night than it is in the day, and any single day/week/month/year is usually at least a little bit warmer or colder than the day/week/month/year before it.

If, like most people, you use your heating system to keep your building at a roughly constant temperature, the amount of energy that your heating system uses will vary from one day/week/month/year to the next, just like the outside air temperature does.

Heating degree days are a simple way to quantify all of this. The idea is that the amount of energy needed to heat a building in any day/week/month/year is directly proportional to the number of heating degree days in that day/week/month/year.

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An example of heating degree days in action

Let me introduce you to a man called Dan. Dan is the facilities manager of an office building, and he's under big pressure to reduce the building's energy consumption. The company CEO, Jock, has noticed the rising cost of energy, and he's decided that the business could, and should, save some money by becoming more energy efficient. Jock hasn't given much thought to how they're going to become more energy efficient, but he's certainly putting a lot of pressure on Dan to make it happen...

So, in January 2018, Dan spent a big chunk of his budget on improving the building's insulation. At the time, he was confident that this would seriously reduce the energy it took to heat the building, and that the savings in the energy bill would very quickly pay for the rather hefty capital cost.

Roll forward to January 2019, a year after Dan's big insulation spending spree, and Dan has a decidedly stressed look on his face... Jock, who's a "numbers guy", wants to see some "solid evidence" that Dan didn't "squander the company's hard-earned cash lining the pockets of some fly-by-night jokers". (No offence intended to the insulation industry – Jock just tends to be a little quick to point the finger...)

Anyway, Dan is sweating, and it's not because the building's temperature control is playing up (the well-insulated building is actually helping to keep things at a steady, comfortable temperature). Dan has just added up the heating energy consumption for 2018, and he's somewhat concerned by what he sees:

It's not that there hasn't been an improvement in the heating energy consumption (there has), it's just that Dan was rather hoping for more of an improvement... After spending a small fortune on insulation, he was actually rather hoping for significantly more of an improvement...

Now, it just so happens that, in Dan's neck of the woods, 2018 was quite a lot colder than 2017. Dan is aware of this, and, reluctant to admit that he might have overestimated the energy-saving potential of his insulation idea, he is pinning his hopes on being able to prove that 2018's cold weather was to blame for the disappointing energy savings. Dan tried explaining this theory of his to Jock, but he was met with a rather blunt "Don't you try fobbing me off with any of your hand-waving nonsense!"

Shame on you, Dan, for forgetting that Jock is a numbers guy...

Fortunately all is not lost, as a colleague has tipped Dan off to these things called heating degree days. They're basically a measure of how cold the temperature was, but they're specifically for heating – if you've got 10% more heating degree days in any given day/week/month/year you should expect 10% more heating energy consumption in that day/week/month/year, all else being equal. (And with the caveat that you have to get the heating degree days in a "base temperature" that makes sense for your building – I'll cover that shortly.)

So, Dan hunted the web until he found Degree, a site that generates degree days for locations around the world. He found a good weather station near his office building and downloaded a few years' worth of heating degree days for that location, in a base temperature that made sense for his building (I'll cover this in a moment). He quickly assembled the following figures:

Applying some simple arithmetic:

Comparing these two figures, Dan concluded that the heating energy efficiency in 2018 was around 20% better than that in 2017. Well done Dan: your insulation plan was a good one, and the company should make good savings from it for many years to come. In fact, it was such a good idea, Jock has convinced himself that it was his idea all along... So, Dan, it's looking unlikely that your insulation success will help your bonus, but at least you can stop sweating – your job security is no longer in immediate danger.

(Advanced note: the kWh-per-degree-day calculations above are a very simple way to estimate energy savings. I don't want to overload you in an introductory article, but, just so you know, it's usually better to use a more sophisticated regression-based approach to calculating energy savings. I suggest you keep reading this introductory article for now, but remember to read about that better approach when you are ready to do similar calculations of your own.)

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So how are heating degree days calculated?

Well, to calculate heating degree days perfectly requires vast quantities of temperature data – infinite quantities to be precise. It's called the "Integration Method" and I'll explain it shortly.

Of course it's not realistic to expect to be able to source infinite quantities of temperature data, so practically speaking the most accurate way is to apply the same Integration Method to detailed temperature readings taken throughout each day (e.g. every 30 or 60 minutes). Given the limitations of weather-station thermometers, 60-minute data is almost as good as infinite data anyway. (Degree has a page on degree-day calculation that explains this in more detail, but I'd recommend you stick with this introductory article for the moment.)

Also, because detailed temperature records (such as 30- or 60-minute data) can be difficult to source, store, and process, a number of "approximation methods" exist to estimate the degree days using less detailed temperature data such as daily maximum/minimum temperatures.

Irrespective of the exact calculation method used, we always start with a base temperature:

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The base temperature of a building

With regard to heating degree days, the base temperature of a building is the outside air temperature below which that building needs heating.

Let's consider a regular office building. In fact, seeing how I put all that effort into The Tale of Dan the Facilities Manager (see above), let's consider Dan the Facilities Manager's office building.

Dan tries to keep his office building heated to around 20°C (about 68°F) – after many years on the job he has determined that this is the temperature at which he gets the least number of people complaining that it's too hot or too cold.

On a summer day, when the outside temperature is 20°C (about 68°F) or above, as you can probably guess, Dan switches the heating off – there's no point in heating a building when it's already warmer than the temperature you want it.

In fact, Dan has figured out that he can switch the heating off when the outside temperature reaches 17°C (62.6°F) – a few degrees below the desired inside temperature. The office has a lot of warm people in it and a lot of warm office equipment too – this essentially provides a few degrees of free heating. In technical terms, this would be described as an average internal heat gain of 3°C, or 5.4°F.

So, when the outside temperature is below 17°C (62.6°F), the heating needs to be on, and when the outside temperature is above 17°C (62.6°F), Dan can switch the heating off without incurring any more complaints than usual about it being too cold. Of course more people might complain that it's too hot, but that's a different story.

What this means is that the base temperature of Dan's building is 17°C, or 62.6°F.

All buildings have a base temperature – it varies from building to building, but you can think of it as depending on two things:

  1. What temperature is the building heated to? (e.g. Dan's building is heated to 20°C or 68°F.)
  2. How much free heating comes from the people and equipment inside the building? In other words, what's the average internal heat gain?

The base temperature of your building will determine the base temperature of the heating degree days that you should use to do your calculations.

(Advanced note: In reality it's more complicated than that – the best base temperature to use can also depend on the thermal properties of the building, the heating schedule, and external influences like solar gains. Considering the two factors above can help you get a reasonable first estimate, but at some point you should read this article on choosing base temperatures for more detail. If you've got good records of the building's energy consumption, it's also a good idea to estimate the base temperature experimentally using regression analysis, and Degree has a regression feature that can do this for you automatically.)

Anyway, that explains the base temperature... But what do the heating-degree-day numbers actually mean? To understand this, you need to have a rough idea of how the figures are calculated.

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Simple example heating-degree-day calculations

With appropriate use of scary-looking formulae, it's quite possible to make it look as though the process of turning temperature readings into degree days is a complicated black box that only the experts can understand. But it's actually straightforward. Well, in practice it is complicated by the large amounts of data and the fact that real-world temperature records tend to be a lot messier than would be ideal (and the more you work with them the more you realize this), but the process is conceptually straightforward at least.

There's rarely a need to calculate degree days yourself (as you can typically just download them from Degree or similar), but understanding how they are calculated makes it a lot easier to understand how to make good use of them.

I'm going to use a few example calculations to explain how the calculation process works for heating degree days. Once you understand the process for heating degree days you will find it very easy to understand the process for cooling degree days too.

Let's say we're dealing with a building with a base temperature of around 17°C (I'm going to stick with Celsius for this explanation, otherwise it'll get really confusing). It's the start of July – technically it's the middle of summer, but the inhabitants of the building are still waiting for "the real summer" to arrive. (I'm from the UK, where it's depressingly common to spend most of the summer months waiting for "the real summer" to arrive...)

Anyway, consider a single day, let's say July 1st, when the outside air temperature was 16°C throughout the entire day. A constant temperature throughout an entire day is rather unlikely, I know, but degree days would be a lot easier to understand if the outside air temperature stayed the same... So, throughout the entire day on July 1st, the outside air temperature (16°C) was consistently 1 degree below the base temperature of the building (17°C), and we can work out the heating degree days on that day like so:

1 degree × 1 day
    = 1 heating degree day on July 1st

If, on July 2nd, the outside temperature was 2 degrees below the base temperature, we'd have:

2 degrees × 1 day
    = 2 heating degree days on July 2nd

Let's look at July 3rd – this was a hotter day, and the outside air temperature was 17°C, the same as the base temperature (i.e. 0 degrees below the base temperature). This gives:

0 degrees × 1 day
    = 0 heating degree days on July 3nd

On July 4th it was warmer again: 19°C. Again, the number of degrees below the base temperature was zero, giving:

0 degrees × 1 day
    = 0 heating degree days on July 4th

You might have guessed: when the outside air temperature goes over the base temperature, you don't get any heating degree days. This makes sense, because you wouldn't need any heating either.

Right, now let's make it a little more realistic. July 5th had a temperature of 15°C from 00:00 to 12:00, and 16°C from 12:00 to 24:00. So for that day we have:

(2 degrees × 0.5 days)
        + (1 degree × 0.5 days)
    = 1.5 heating degree days on July 5th

(The 2 degrees is because 15°C is 2 degrees below the base temperature of 17°C, and the 0.5 days are because 00:00 to 12:00 is half a day. We calculate the heating degree days for each period in the day, and then add them together to get the total for that day: 1.5.)

On July 6th, colder weather started moving in: the temperature was 16°C from 00:00 to 06:00, 15°C from 06:00 to 12:00, 14°C from 12:00 to 18:00, and 13°C from 18:00 to 24:00. This gives the following:

(1 degree × 0.25 days)
        + (2 degrees × 0.25 days)
        + (3 degrees × 0.25 days)
        + (4 degrees × 0.25 days)
    = 2.5 heating degree days on July 6th

Now, on July 7th, the temperature just kept changing... Like it might on a real day... Between 00:00 and 00:30 it was 13°C, between 00:30 and 01:00 it was 12.9°C, between 01:00 and 01:30 it was 12.9°C, between 01:30 and 02:00 it was 12.8°C... It started getting warmer around 05:00, peaking at 17°C between 14:00 and 14:30, and dropping again until it reached about 13.7°C between 23:30 and 24:00. Complicated!

A proper calculation would not make for particularly interesting reading, so I'll leave most of it out. But essentially you just have to add up the figures for each of the half-hour periods in the day (one half-hour period is 1/48 days):

(3 degrees × 1/48 days)
        + (3.1 degrees × 1/48 days)
        + ....... etc.
    = 1.9 heating degree days on July 7th

Hopefully by now you're getting the idea!

So, from the examples above we've got:

We'd expect the heating energy consumption on each of those days to vary with the heating degree days (assuming we chose an appropriate base temperature for the building). So, the heating on July 2nd would use twice as much energy as the heating on July 1st, and on July 3rd and July 4th the heating wouldn't use any energy at all (zero degree days on those days would mean it would be warm enough that the heating would never have switched on).

One of the best things about degree days is that you can add them together to get figures for longer periods. Adding together the figures above gives a total of 8.9 heating degree days for the week beginning on July 1st and ending on July 7th. So we'd expect that the heating system would have used 8.9 times more energy in that whole week than it used on July 1st alone.

If you've got daily heating-degree-day values for each day in a month, you can add them up to get the total heating degree days for that month. If you've got the heating-degree-day values for each month in a year, you can add them up to get the total heating degree days for the whole year.

And therein lies what I consider to be the beauty of degree days: you can add them up to get a total for a long period of time, and that one total figure will represent all the relevant variations in temperature over the period. (Contrast that with, say, an annual average temperature, which tells you nothing about how the temperature varied within the year.)

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Calculating heating degree days with real-world temperature data

The calculation method that I explained above is known as the "Integration Method" and it is the correct method for calculating heating degree days. For each period over which the outside air temperature was constant, you multiply the degrees below the base temperature by the number of days that the temperature was constant for (usually a small fraction of a day), and then you sum all the values together to get the total heating degree days for the longer period that you are interested in (e.g. a day, month, or year).

In the real world, outside air temperature doesn't remain constant – in fact it changes pretty much all the time. A pedantic mathematician could argue that you'd need an infinite number of temperature readings to calculate degree days properly. But in reality half-hourly or hourly temperature readings are plenty good enough to calculate degree days accurately using the Integration Method described above.

To turn real-world temperature data into degree days you would typically assume a straight-line variation between each temperature reading. It's basically just joining the dots, but you could call it "linear interpolation" if you wanted to sound more technical. You can see it in the example chart below (which assumes a base temperature of 14°C):

Calculating heating degree days

You would then split the day into sections (hours in the example above), work out the degree days for each section (basically working out the orange-shaded area above the line), then sum the figures together to get the total for the day (the total orange-shaded area). It gets a bit more complicated when the temperatures aren't perfectly regular and when the temperature crosses your chosen base temperature, but the theory is the same. Degree has a page on calculating degree days that goes into more detail if you want it, but understanding the general concept is probably enough for most people.

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Approximation methods

The Integration Method described above is the accurate way to calculate degree days, but there are also approximation methods that estimate the degree days from less-detailed temperature data that is easier to collect, store, and process. They were mostly developed a long time ago when many weather stations only recorded daily maximum and minimum temperatures, and before computers made it much easier to store and process large quantities of data.

Approximation methods can never be as accurate as the Integration Method, because, without using detailed temperature records, it is impossible for them to fully capture the temperature variations that occur within each day. The better approximation methods do manage to come pretty close on most days in most climates, but I would still recommend using accurately-calculated data whenever possible.

Degree has a page on calculating degree days that goes into more detail on approximation methods, but the summary info above is probably enough for most people.

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Celsius-based degree days and Fahrenheit-based degree days

The examples above use Celsius temperature readings and base temperatures to give "Celsius-based degree days". This is the norm outside the US.

In the US it's more common to use "Fahrenheit-based degree days". These are calculated in exactly the same way, just using Fahrenheit temperature readings and base temperatures instead of Celsius. As weather stations typically record in Celsius, to calculate Fahrenheit-based degree days you'll usually have to convert all the recorded temperature readings into Fahrenheit first.

Fahrenheit-based degree days are 9/5 times bigger than Celsius-based degree days with an equivalent base temperature. So, for example, 500 Celsius-based degree days with a base temperature of 15°C are equivalent to 900 Fahrenheit-based degree days with a base temperature of 59°F. (Note you don't add or subtract 32 because degree days are effectively zeroed at the base temperature, whether Celsius-based or Fahrenheit-based.)

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A note on common base-temperature confusion

Unfortunately, when it comes to degree days, confusion is abundant, even amongst those who really should know better... So a lot of people are unaware of the fact that degree days can come in any base temperature, and that typically you should choose the base temperature to fit the building that's energy consumption you're looking at.

Earlier I gave the example of a particular office building with a base temperature of 17°C (62.6°F). This is probably a fairly common base temperature for an office building, although the actual base temperature of any particular office building will depend on the temperature it's heated to, and the people and equipment that are in it.

If your building happens to be a swimming pool hall, the base temperature might be more like 26°C (78.8°F), and you'd want to use a very different set of heating degree days for your calculations. Or maybe you have a just-warm-enough-to-prevent-a-lawsuit factory, or a house that you like to keep uncommonly warm – whatever the building, it will have its own base temperature, and you should use heating degree days with that base temperature when you're looking at its energy consumption.

So it's a popular misconception that degree days come with a single base temperature. In the US, this is typically quoted as being 65°F, whilst in the UK it's typically quoted as being 15.5°C. I suspect this stems from the days when degree days were made available in magazines and the like, and there was no more room in magazines to fit data with a range of base temperatures than there was the computer technology to calculate and store all that data in the first place.

So the next time you hear something along the lines of "heating degree days have a base temperature of 65°F", you will hopefully know better than to believe it!

(All that said, it does make sense to use a consistent base temperature when you're comparing the climate of one location with that of another. But, when you're looking at the energy consumption of a building, there's no doubt about it: you should use degree days with the most appropriate base temperature for that building.)

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Cooling degree days

Cooling degree days are a measure of how much (in degrees), and for how long (in days), the outside air temperature was above a certain level. They are commonly used in calculations relating to the energy consumption required to cool buildings.

I think of them as heating degree days in reverse: whilst heating degree days start adding up when the outside air temperature drops below the base temperature, cooling degree days start adding up when the outside air temperature rises above the base temperature. So the base temperature of cooling degree days is just the outside air temperature above which the building needs cooling. Pretty straightforward, right?

Here's a graphical example showing how cooling degree days are calculated using temperature data from a weather station that records approximately every 20 minutes:

Calculating cooling degree days

Degree's page on calculating degree days explains this example in more detail, but, as for heating degree days above, a conceptual understanding is enough for most people so I wouldn't recommend digging into the details unless you are particularly keen.

One important point to note though: for a building with heating and cooling, the optimal base temperature for cooling degree days (the "cooling base temperature") is usually different (typically higher) than the optimal base temperature for heating degree days (the "heating base temperature"). Degree has some useful guidance on choosing base temperatures that explains this and that should help you choose appropriate base temperatures for your building(s). It's well worth reading.

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Growing degree days

In their simplest form, growing degree days are calculated in the same way as cooling degree days, but the base temperatures used are the temperatures above which certain plant or insect growth occurs.

Different plants have a different base temperature above which they will start to grow, and their growth will typically be roughly proportional to the amount by which that base temperature is exceeded. This is similar to the way in which building cooling is proportional to the amount by which the building base temperature is exceeded.

Unlike heating and cooling degree days, growing degree days sometimes have cut-off points (e.g. when the temperature gets over 65°F we stop counting – that sort of thing). To be perfectly honest, my expertise are in the heating and cooling side of degree days, so I won't try to elaborate much further on growing degree days!

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Further degree-day advice

Now you've finished this introduction to degree days you should have a pretty good idea of what degree days are, why they are useful, and roughly how they are calculated. (You are unlikely to need to calculate them yourself as it's much easier to download them from Degree, but it is still important to understand where they come from.)

This is not the end of the story, however, as there is a lot more to using degree days effectively than the basics explained in this article. This is actually just the first of several articles about degree days and how best to use them. I suggest you read the following three in particular:

Best of luck with all your degree-day analysis, and I hope it helps you save energy!

Martin Bromley, Degree

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As mentioned above, this article was originally published as a Google Knol. It received a lot of comments there before Google discontinued the service. We decided not to copy all the comments here because, although they were greatly appreciated at the time, they were all either compliments on the article (and we don't wish to encourage ongoing swelling of the author's ego) or questions that we have either edited the article to answer better, or answered better elsewhere on this site. But a big thank you to everyone who commented originally, and please feel free to contact us if you have any questions that aren't answered in the articles above or in our FAQ.

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