Why 5000+ Energy Pros Get Data From Us Each Month...
New custom breakdown option! Now it's easy to deal with irregular utility bills... Just copy/paste in your dates from Excel and download degree days to match. Select "Custom" below to try it out.
Degree Days.net is aimed at the energy-saving professionals 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).
Degree Days.net calculates its degree days using temperature data from Weather Underground, a weather-data service with 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 across 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.
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 that 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 judgement 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.
Some of the stations are listed with additional information: a blue bar indiciting 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 pop up 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.
Some day we hope to have bars and stars for all of the weather stations. But there are technical challenges that are preventing us from reaching that goal just yet. But we have got bars and stars for most of the better-quality stations, so it usually makes sense to focus on the stations with bars and stars first.
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.
Weather Underground is a US company, and it holds data from 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.)
There are a lot of UK weather stations stored in the Weather Underground database. In our experience, searching for city name and country name together works well (e.g. "London, UK"). The database doesn't currently hold postcode information, so there's no point in searching for that.
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. Some countries are very well covered, although availability does vary greatly 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.
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 three or four letter international airport code of the airport that's closest to you. Airports tend to be particularly well represented in the Weather Underground database - there are lots of them and most of them 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!
Degree days have traditionally come in a limited range of base temperatures such as 15.5°C, 18.5°C, and 65°F. It is 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 calculation approaches explains this issue in much 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. This makes it easy for you to estimate an appropriate base temperature for your building, and then refine your choice by correlating your kWh energy-consumption figures with the base temperatures nearby until you find the best fit (see this article on linear regression analysis for step-by-step instructions).
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. 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).
Degree days are unfortunately rather easy to use badly (in a way that leads to erroneous and misleading figures), and rather difficult to use well. It can also be hard to turn degree-day-based analysis into actionable tasks that reduce consumption. There are better ways for beginners to get started saving energy (like, for example, obtaining and 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 that are already experienced in using degree days as part of a broader energy-management strategy.
However, if you are a degree-days beginner that isn't afraid to delve further into a deceptively complicated form of data analysis, please read on:
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.
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 that 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 that 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.
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?!