Degree Days Calculated Accurately for Locations Worldwide

Buildings require more heating in colder weather, and more air-conditioning in hotter weather. Degree Days.net provides the data to quantify this and help monitor, manage, and reduce energy consumption in millions of buildings around the world. It is developed and maintained by BizEE Software.

We're thinking about a new feature: A few people have asked us for the ability to generate degree days within a temperature band, like HDD with a base temperature of e.g. 18°C but only for periods when the outside temperature was between e.g. 0°C and 10°C. It seems like a very specialist feature, so we want to know how many people could use it, and why, before we build it. Please email us if it would be useful to you!

If you are already experienced with degree days, you will probably find most of the options above to be self explanatory (the less-obvious regression options have separate instructions). However, we suggest you read the tips below as they do cover some important points.

If you are new to degree days, you might want to skip straight to the brief introduction at the bottom of this page. You might also want to find out why 5000+ energy professionals get data from here each month (and often a lot more frequently).

Choosing the best weather station for location and accuracy

Degree Days.net calculates its degree days using temperature data from thousands upon thousands of weather stations worldwide.

The closest weather station isn't necessarily the best choice, because weather stations vary considerably in their data quality and coverage (the length of time covered by their historical records). Many stations in our system have a long history of reporting frequent, regular, accurate temperature readings, from which we can calculate high-quality degree days going back a long time. But lesser stations may have shorter data histories (e.g. because they were only set up recently), gaps and errors in their reporting, and less-than-ideal recording frequencies (for example, some only record the temperature every 6 hours or so, and some don't record at night).

We have programmed Degree Days.net to make the best of whatever data is available. We use sophisticated processes to identify and discard erroneous temperature readings and to fill gaps with estimated data. We mark all affected figures with a "% estimated" value so you can assess the impact of the problems detected.

When you search for a location, you will typically get a long list of weather stations to choose from. You can click the "map" link to see where they are all located, and each listed station will have a blue bar indicating how far back in time the data can go (the coverage), and a star rating indicating the estimated quality of the data (based on the "% estimated" values mentioned above). You can hover your mouse over any station to see a popup with more specifics.

We aim to list the best weather stations for your search location first, considering the data quality and coverage of each station, as well as the distance from your search location. So it's generally better to choose from the top of the list rather than the bottom.

Degree Days.net is not unique in having to deal with less-than-perfect temperature data, as pretty much all real-world weather stations have at least minor problems occasionally. We do what we can to correct for these problems (we are obsessive about calculating degree days as accurately as possible), but we also feel it's important for us to highlight the underlying data-quality issues, to help people decide which weather stations to use, and assess the accuracy of their data analysis.

You should use your own judgement when deciding whether or not to use data from any particular station – only you can determine the level of accuracy that you require. Also, please bear in mind that even though Degree Days.net has been carefully programmed to detect errors in the source temperature data and to minimize the chance of calculation errors, we cannot vouch for the accuracy of any data that Degree Days.net generates.

How can I get data for my "degree-day region"?

The practice of splitting a country into "degree-day regions" stems from a time when degree days were disseminated in print publications with limited space. The degree days for a "region" were typically just the degree days for a "reference" weather station that had been chosen to represent the region. If you can find out what the reference station for your region was, you should be able to get data for it (and therefore for your region) from our system.

However, the internet has made it feasible to make much larger quantities of data readily available, and there is little need for degree-day regions nowadays. It is usually better to choose a good local weather station, as this should represent the weather (and energy consumption) at your building better than any "reference" station further away.

Searching for weather stations in the US

Our database covers a huge number of US weather stations. You should have no trouble finding nearby stations by searching for your city, state, or zip code.

(US visitors might be interested to know that the 10 most popular US states for degree-day downloads are, in descending order of popularity: New York, California, Massachusetts, Pennsylvania, Texas, Ohio, Michigan, Maine, Connecticut, and Florida. And the most popular US cities are: Boston MA, Philadelphia PA, Denver Colorado, New York NY, Atlanta Georgia, Minneapolis Minnesota, Phoenix Arizona, Houston TX, Detroit MI, and Portland ME.)

Searching for weather stations in the UK

There are lots of UK weather stations stored in our database. In our experience, searching for city name and country name together works well (e.g. "London, UK"). You can also search for UK postcodes.

Searching for weather stations elsewhere

After the United States and the United Kingdom, the 10 most popular countries for data downloads are, in descending order: Canada, Spain, Ireland, Germany, France, India, Italy, Australia, Greece, and Sweden. India's coverage is patchy (although there are some good stations dotted about), but the rest of those popular countries, and many others (particularly in Europe), have good weather stations in the more populated areas at a minimum. Many countries are very well covered, although availability does vary from country to country.

We suggest you try searching for city name and country name together, using anglicized names (e.g. "Copenhagen, Denmark", or "Amsterdam, Netherlands"). If searching for city and country doesn't work, try searching for city name alone. Postal code searches work well for most countries too.

If you're struggling to find a good weather station...

If your search location isn't recognized, check the spelling carefully and try alternative spellings if you can think of any, as the search facility can be quite particular. You could also try searching for towns and cities nearby. If you can get results for somewhere close you can click the "map" link and then focus in on the stations around the location you really want.

If you are struggling to find good stations in remote locations, or in regions with poor weather-station coverage (like parts of Africa, for example), you could search for larger cities further afield until you find something acceptable.

You can also search for four-letter ICAO airport codes. Most airports have a weather station, and many have high-quality weather data going back a long time. WMO IDs work too, though for stations with both a WMO ID and an ICAO code, our system will use the ICAO code.

Any files of data that you have downloaded previously will contain the ID of the weather station you used. You can use that ID again each time you want fresh data. Though if your weather station stops working (which does unfortunately happen occasionally, especially with lower-quality stations), you will have to find an alternative.

If you are really stuck, feel free to email us to see if there is anything we can do to help. We are not magicians (unfortunately!) but sometimes there is something we can do to get a troublesome station working again or to add in a station that our system had previously overlooked.

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Choosing appropriate base temperatures

Degree days have traditionally come in a limited range of base temperatures such as 15.5°C, 18.5°C, and 65°F. But it's rare for real-world buildings to align accurately with any of these pre-prescribed base temperatures, and degree days with an inappropriate base temperature are a significant cause of inaccuracy in calculations relating to weather-dependent energy usage. Our article on the problems with common approaches to degree-day analysis explains this issue in more detail.

Degree Days.net will generate degree days to any base temperature you choose. If you check the box to "Include base temperatures nearby", Degree Days.net will calculate your degree days to a range of base temperatures around the one that you specify.

For most analysis, it's important to use degree days with a base temperature that makes sense for the building you are investigating. And buildings vary a lot. The following two resources should help you choose appropriately:

• Our article on choosing base temperatures should help you understand the key factors to consider and make a rough estimate based on your knowledge of the building.
• Our regression tool will test regressions of your energy-consumption data against degree days with different base temperatures to find the ones that give the best statistical fit. It is accessible via the web tool above by choosing "Regression" as the data type and following the instructions from there.

Used together, these two resources should help you choose base temperatures that fit your data and make logical sense for your building. Your subsequent analysis should be considerably more accurate as a result.

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Generating data to match your utility bills or fuel consumption records

Gas and electricity bills often don't line up neatly with calendar weeks or months. And it's rare for records of oil, LPG, or wood-burner consumption to fit the calendar.

But irregular dates are not a problem: you can specify your dates as a custom breakdown and Degree Days.net will generate the degree days to match. Just choose "Custom" as the breakdown and follow the instructions from there. Alternatively you can get daily degree days and sum them together to get totals for each period of consumption that you have records for. This is a perfectly valid thing to do and will give you the same figures (albeit with a little more spreadsheet work).

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More information for energy professionals

As an energy professional you don't need us to explain what degree days are or their importance in analysis of HVAC energy consumption. However, unless you are already very experienced with degree days, we are confident that you will find some value in our articles on degree days and how best to use them. These include the articles mentioned above, but they also cover regression analysis (which is core to most effective degree-day analysis), how to calculate energy savings after making changes to reduce consumption (e.g. after installing new insulation), and more.

We would welcome your feedback on anything important that is missing or anything we could improve upon. The data is our main focus, but we do try to provide good guidance as well!

Finally, although we have designed the free website above to be enough for basic needs, we also aim to cater for the more sophisticated needs of many of the energy professionals, multi-site organizations, academic/government researchers, and energy-software developers who use our system. If you could benefit from additional data, data for lots of locations, or automated access to data (in large or small quantities), please take a look at our overview of Degree Days.net products to see how they can help.

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Degree days for beginners

Degree days are key to reliably accounting for the weather in analysis of building energy consumption. However, it is easy for beginners to use them inappropriately, in ways that can lead to erroneous and misleading figures. And, without experience, it can be hard to turn degree-day analysis into actionable tasks that reduce consumption. There are often simpler ways for beginners to get started saving energy (like, for example, using a Kill-A-Watt meter to find electrical waste, and a cheap infrared laser thermometer to find thermal leaks). For these reasons, Degree Days.net is primarily aimed at the energy-saving professionals who are already experienced in using degree days as part of a broader energy-management strategy.

However, if you are a degree-days beginner who isn't afraid to delve further into this useful form of energy data analysis, please read on:

So what are degree days?

Degree days are essentially a simplified representation of outside-air-temperature data. They are widely used for calculations relating to the effect of outside air temperature on building energy consumption.

"Heating degree days", or "HDD", are a measure of how much (in degrees), and for how long (in days), outside air temperature was lower than a specific "base temperature" (or "balance point"). They are used for calculations relating to the energy consumption required to heat buildings.

"Cooling degree days", or "CDD", are a measure of how much (in degrees), and for how long (in days), outside air temperature was higher than a specific base temperature. They are used for calculations relating to the energy consumption required to cool buildings.

Degree days also have applications relating to plant growth ("growing degree days"). However, our focus is on making software for energy saving, so our expertise are in the energy-saving applications of heating and cooling degree days.

Further information for beginners

We suggest you start with our introduction to degree days – it's a nice, easy-going explanation of degree days and what they're used for.

Following that we suggest you work through our recommended articles on degree days and how to use them effectively. These are aimed more at energy-saving professionals than beginners, so you might find them a little heavy going, but they do try to explain the basics as they go along. If you can set aside the time to work through them slowly, they should give you a lot of useful practical information on how to use degree days in your energy data analysis.

If you're enthusiastic about energy in general, you might also be interested in the information we've compiled about energy-related degree programs and courses. It's something of a work in progress, but we already have a lot of interesting-looking academic courses listed.

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