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 School of Geography and the Environment

Degree Days for energy management

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Free building energy monitoring software

The Environmental Change Institute (ECI), based at the University of Oxford, has developed SMEasure: a new easy-to-use building energy energy management system for small and medium businesses to measure, benchmark and target the energy use of your buildings simply by entering regular gas and electricity meter readings. Read more about SMEasure and how to join up.

Free world degree day data

Free prelimanary cooling degree day and heating degree day data available for worldwide stations, calculated for and updated weekly. Data is preliminary and feedback is requested. For data please visit World Degree Day Data

Houses

Heating energy use depends largely on the external temperature and therefore needs to be accounted for in order to properly monitor a building’s energy performance. This is important to see whether the heating system is behaving properly and to judge and benchmark the energy use of a building normalized to some base weather. The integrated difference between the external temperature and the building base temperature where the external temperature is below the building base temperature gives the ‘heating degree days’ for a building.

A well run building should have an energy usage which is proportional to the number of degree days for some time period. The slope of the plot of energy use versus degree days (the performance line) has a slope which is the building energy loss rate. The performance line crosses the y axis at the building base energy use (energy use which is independent of heating). The building base temperature is crucial to a rigorous analysis using degree days.

The physical meaning of the building base temperature is the external temperature below which a building needs to be heated. Mathematically, it is the building set point temperature (or for intermittently heated buildings the mean 24 hour temperature) minus building internal gains divided by the building energy loss factor. The value has for generations been taken as 15.5 degrees or 18.5 degrees for hospitals.

However, it can and does vary greatly and can be as low as 10 degrees for a thermally well insulated building with large internal gains. Use of the wrong base temperature ruins the linearity of a well run building and will make benchmarking erroneous.

The Carbon Trust has a document called "Degree Days for energy management" which discusses the approach in detail.

Use of Met. Office Weather Stations

The best degree day data to use would be accurate data at the position of the building of interest. Measurement and logging of accurate hourly temperatures is not a simple task and the placing of electronic thermometers and the use of a Stephenson Screen (http://en.wikipedia.org/wiki/Stevenson_screen) would be crucial to achieve data consistency. In most cases, it is best to use Met. Office weather data. The disadvantage of being some distance from the building of interest is overwhelmingly outweighed by having temperature data that is properly collected, calibrated and checked for errors. Importance of hourly temperature readings

Degree day data has traditionally been calculated from the ‘British Gas’ equations first formulated in 1928 and applied to minimum and maximum daily temperatures. It attempts to estimate the integral between the external temperature and the building base temperature. This method, good for when only minimum and maximum daily data was available, is now largely outdated due to the existence of rich hourly meteorological datasets. These hourly datasets calculate the exact value of the integral to find the degree days and there is a considerable error between the two methods (see Day and Karayiannis, Identification of the uncertainties in degree-day based energy estimates, BSER&T, Vol. 20, No. 4, pp 165-172, December 1999 and Day AR and Karayiannis TG, Degree Days: Comparison of calculation methods, Build. Serv. Eng. Res. 19(1), 7-13. 1998).

Description of files

Weekly and daily updated data is derived from 77 Met. Office weather stations (view locations of the stations on the Met Office website) in the UK. Daily updated data consists of the daily degree days for the past 7 days. Weekly updated data consists of daily, weekly and monthly degree days since summer 2007. The data is given for a range of building base temperatures between 10.5 C and 20.5 C.

The data is derived from the following Met Office data: http://www.metoffice.gov.uk/education/archive/uk/observation_0.html.

The data format is as follows: 1 file for each of 77 weather stations, comma seperated variable files (.csv) (which can be opened directly into Excel), with column 1 being the date (daily data), date of the Monday following the week for which the degree days have been calculated (weekly data) or month (monthly data). Subsequent columns are heating degree days for the day/week/month calculated at the building base temperatures in the column header.

Historical data is Met. Office weather stations derived data supplied through the British Atmospheric Data Centre (http://badc.nerc.ac.uk/home/index.html). The data consists of daily, weekly and monthly heating degree days from Jan 1987 - November 2007 for a range of building base temperatures between 10.5 C and 20.5 C. The data files are supplied as zipped files for each year, daily weekly or monthly. Each zipped file contains around 200 files, 1 for each weather station. The 77 files are a subset of the 200. The file headers all contain the building base temperatures for which the calculations were made. The final column represents the number of hourly readings that were used to make the data. This final column represents and error check on the data consistency showing where data has dropped out. This final column should give the hours in a day, week or month for perfect data.

Feedback about the usefulness of this site and the data it contains will be gratefully received at

Disclaimer

Oxford University does not guarantee any accuracy in the data and it is used at your own risk.

Degree Day Downloads

Weekly Updated Degree Day Downloads

Heating

Cooling

Historic Degree Day Downloads