Hourly temperature data : We are pleased to announce that Degree Days.net is now providing hourly temperature data to Degree Days.net Pro Solo/Team customers and Standard+ API customers. We recommend degree days for most energy-data analysis (and our degree days are already calculated accurately using detailed temperature readings), but hourly temperature data is useful for some specialist calculations. Find out more about Degree Days.net Pro and hourly temperature data here .

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 .

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Degree Days.net is aimed primarily at those who are already experienced in using degree days for energy-related calculations. Provided you fit this description, you will probably find most of the options above to be fairly 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 5,000+ 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.

Ideally you'd use the weather station that's closest in climate to the location of the building that's energy consumption you're analyzing. This should give a better representation of the weather at the building than any "reference" station for the larger region in which the building sits.

However, the best weather station to use often isn't the closest, because data quality and coverage varies considerably between weather stations. Degree Days.net will try to list the stations that it thinks can best represent your search location first. It considers the data quality and coverage of each station, as well as the distance from your search location.

Airport ICAO stations and WMO stations generally have the best quality data. The ICAO stations are at airports and have 4-character IDs; the WMO stations (often also at airports) have 5-digit IDs. Larger airports typically have many years of frequent, regular, accurate temperature readings - ideal for calculating high-quality degree days. Smaller airports and non-airport WMO stations are more variable: some are great, but others have a less-than-ideal recording frequency. For example, some only record the temperature every 6 hours or so, and some don't record at night. The star ratings shown with all ICAO and WMO stations will help you assess their data quality.

generally have the best quality data. The ICAO stations are at airports and have 4-character IDs; the WMO stations (often also at airports) have 5-digit IDs. Larger airports typically have many years of frequent, regular, accurate temperature readings - ideal for calculating high-quality degree days. Smaller airports and non-airport WMO stations are more variable: some are great, but others have a less-than-ideal recording frequency. For example, some only record the temperature every 6 hours or so, and some don't record at night. The star ratings shown with all ICAO and WMO stations will help you assess their data quality. Personal weather stations (PWS) are amateur run, typically installed in people's gardens, though some are run by businesses and other organizations. A minority of PWS have years of high-quality data, and these are usually shown with a long coverage bar and a good star rating (see further below for more on bars and stars). But the majority of PWS don't have a good data history, as people put them up and then move or lose interest and fail to keep them maintained, so their weather reporting becomes patchy or just stops altogether. Degree-day-based analysis usually requires a long, continuous data history, which most PWS don't have. So we generally recommend favouring higher-quality ICAO and WMO stations over PWS, even when the PWS are closer (which is very often the case). But there are some good PWS out there, and they can be useful when there's nothing else around.

We have programmed Degree Days.net to make the best of the data that's available. We use sophisticated processes to identify and discard erroneous temperature readings and to fill in gaps with interpolated data. Figures based on estimated data are marked with a "% estimated" value so you can see where the detected problems lie.

Degree Days.net is not unique in having to deal with less-than-perfect temperature data, as the vast majority of real-world weather stations are less than perfect in some shape or form. But, given the types of analysis for which degree days are used, we feel it's particularly important for us to highlight the underlying data-quality issues, to help people assess the accuracy of their analysis and decide which weather station(s) to get data from in the first place.

Do make sure to use your own judgment when selecting 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 obvious errors in the source temperature data and to minimize the chance of calculation errors, we can not vouch for the accuracy of any degree days that Degree Days.net generates.

Coverage bars and data-quality stars

The vast majority of the weather stations you are likely to use are listed with additional information: a blue bar indicating how far back in time the station's data can go, and a star rating indicating the estimated quality of the data. If you hover your mouse over one of these stations, you should see a popup with more specifics.

For stations with bars and stars, we calculate degree days using a slightly more accurate method than that which we use for stations without. If you are interested, you can read more about our calculation processes here.

All the ICAO and WMO stations have bars and stars, and most of the better PWS have them too. Ideally we'd like to have bars and stars for all the PWS, but there are technical challenges that are preventing us from reaching that goal just yet. But we have got bars and stars for the vast majority of the better-quality stations, so it usually makes sense to focus on the stations with bars and stars first.

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

The practice of splitting a country into a limited number of "degree-day regions" stems from a time when degree days were disseminated in print. Since that time the internet has made it feasible to make much larger quantities of data readily available.

We think that it's generally much better to use degree-day data from a local weather station, as that will typically represent the weather at your building better than the data from 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 a lot 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 that initially you try searching for city name and country name together, using anglicized names (e.g. "Copenhagen, Denmark", or "Amsterdam, Netherlands"). Some countries just don't seem to register despite having stations in the database, so 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 station...

You might try searching for towns and cities near to your location. The search facility isn't perfect, and you'll often find that, whilst a search for one city won't turn up anything useful, a search for a nearby city will.

If all else fails, you should hopefully find success by searching for the four-letter ICAO code of the airport that is closest to you. Most airports have a weather station, and many have high-quality weather data going back a long time.

Once you've found a good weather station near your building, keep a record of the ID - you can use it next time you want updated degree days.

If you discover any other useful tips for weather-station searching, please let us know!

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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 degree-day-based analysis approaches 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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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-based analysis into actionable tasks that reduce consumption. There are often better 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 in the energy industry 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 a really basic introduction, you might like to read this Google Knol about degree days - it's ideal if you're looking for an easy-going explanation of degree days and what they're used for.

If you are interested in finding out more, we suggest you read this article on degree days. It describes degree days and how they are commonly used, and, more importantly, it highlights the critical issues you should really be aware of if you are planning to use degree days in your own energy-related calculations. Although the bulk of the article is pitched more at energy-saving professionals than beginners, it does explain the basics as it goes along.

Be warned, however: you might find it a little heavy going!

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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What next?

And that's about it at the moment - it's mainly about the data after all. If you've not done so already, why not head back up to the top of this page and give Degree Days.net a go?!

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