What are degree days?
One of the major uses of energy in a building is for space heating (and cooling in hotter climates.) The main driver for weather dependant energy use is the external temperature and to a lesser extent the amount of sunshine, humidity and wind. The first task when analysing any energy use is to separate the weather dependant energy use from the baseload use and plot it against what we think is the main driver of this energy use.
For this we need to be able to quantify how hot or cold a particular week has been. Heating degree days (HDD) are quantitative indices designed to reflect the demand for energy needed to heat a building. Heating degree days tell us to what extent and for how long the external temperature has been below the building base temperature.
The most technically correct way of calculating HDD is to add up the number of degrees below the base the external temperature is at each hour in the day and divide by 24. Even better would be to add up at minutes or seconds, but the difference is negligible and most weather stations report hourly). Heating degree days include only hours where heating is needed and take account fundamentally of the shape of the external temperature over time.
A similar index, cooling degree days (CDD), reflects the amount of energy used to cool a building. Cooling degree days analysis is not used sMeasure in the UK.
- How do we calculate heating degree days?
- What is the building base temperature?
- How are degree days used in sMeasure analysis?
How do we calculate heating degree days?
Our degree days data is derived from hourly temperature observations from these 77 weather stations across the UK.
Before the advent of modern communications and computing, these hourly temperatures were unavailable. What existed were minimum/maximum mechanical thermometers which recorded the previous day’s high and low temperature. Traditionally the average has been used in building energy management because this is what has been available, but it presumes no heating necessary when the temperature is below the base temperature if at another time it is above.
An improvement came with Met Office equations which are used in the UK. This analysed the diurnal cycle of many temperature profiles and came up with an equation which approximated the actual degree days from the min/max temperature by assuming a standard shape. This was a great improvement but still an approximation to the actual degree days.
We have created a degree dataset based on peer reviewed academic work and this dataset is integrated into our software to offer the best weather analysis. We only ever calculate degree days from hourly integrals. The large errors that are introduced by other methods as discussed in one of our papers.
We make available historical and present HDD and CDD for each of these 77 weather stations. You can download this data from us here.
What is the building base temperature?
In the case of heating of a building, the energy flow both through the building fabric and through normal air changes/ventilation is proportional to the difference between the internal and the external temperature. A building that wants to maintain an internal temperature of, say, 21C doesn’t need heating whenever the external temperature is below 21C because there are also heating gains provided by sunlight through windows, heat from devices, appliances and lights and even heat that comes from the people inside. The external temperature below which we need to heat a building is dependent on the building fabric, air-tightness and additional gains. It is generally several degrees below the internal set point temperature. It can be many degrees below for very well insulated buildings with lots of people and equipment.
More information on building base temperature
How are degree days used in sMeasure analysis?
Degree days are used to create the performance chart for your building. By plotting the number of degree days each week against your energy use for that week it is possible to determine the baseload and degree day dependence of energy use in your building.
Amongst other things this chart will:
- Detect if abnormal energy consumption might be caused by faults in your heating system
- Identify if your energy consumption is abnormal to the seasonal weather pattern
- Extrapolate annual energy consumption from a shorter period of monitoring
- Help you plan appropriate capital investment for upgrading the building fabric to save energy
It is important that you set the closest local weather station for each of your buildings to get the most accurate analysis.