Degree Days - definition

The method of degree days or Accumulated Temperature Difference - ATD allows the need for heating in a building to be assessed.

It is usually based on standard assumptions:

• Firstly, on average, what is the difference in temperature between inside and outside of an un-heated building ? (E.g. in UK this is approximately 3 degrees centigrade.)
• What is the internal design temperature ? (Say 18.5 degrees centigrade.)

Hence the base temperature can be calculated:

• base temp = design temp - avg temp difference (inside & outside)

So, conventionally, Tb = 15.5 degrees centigrade in the UK.

For locations where the outside temperature is less than the base temperature, the following formula may be applied:

• Degree Days = Tb - (Tmax+Tmin)/2

Where Tmax and Tmin refer to daily outside temperatures.

For example:

If the daily average outside temperature (i.e. (Tmax+Tmin)/2) is 1 degree below Tb, the Degree Days = 1. If the daily average outside temperature is 2 degrees below Tb, the Degree Days = 2.

The use of Degree Day values:

The Degree Days value may be summed over several days, or over a month, or over the whole Winter season. This accumulated temperature difference total for a locality is a measure of climatic severity. It can be used in the calculation of heat loss and energy consumption.

Limitation of published Degree Day data:

For example, the UK Government publishes Degree Day data averaged over several years for locations across the country. However this is based simply on Meteorological Office MET-Station temperature measurements and therefore does not account for extra heat losses due to exposure to wind, and solar heat gains.