2.4.2 Degree days

We can also use this heat loss coefficient together with the number of degree days to understand how much space heating energy a building might use in different locations. Over a long period, such as a day or so, the heat loss from a building will be proportional to the average temperature difference between the interior and the outside air. If on a given day the average internal temperature was 20°C and the average external temperature was 10°C, then the difference would be 10°C. We would describe that particular day as having ‘10 degree days’. If on another day the average internal temperature was the same and the external temperature was zero, 0°C, i.e. an average difference of 20°C, we would describe that day as having 20 degree days and expect the building to lose twice as much heat as on the first day.

However if the average external temperature was higher than the interior, then there would not be any heating requirement, and the number of degree days would be zero (rather than a negative number). The total heating requirement over a month will be proportional to the sum of all the degree days of the individual days.

Table 8 gives some long-term averages for sample UK locations. Given their long history of use, it is not surprising that they are normally produced in the UK with a standard indoor base temperature of 60°F, equivalent to 15.5°C.

Table 8 gives an annual total of 1889 degree days for the London area. A first estimate of an annual heating energy consumption of our house in watt-hours would be the heat loss coefficient, 127.3 W K^–1, multiplied by the number of degree days multiplied by 24 (to convert from days to hours). Dividing by 1000 then gives the result in kilowatt-hours (kWh).

• Annual consumption = 127.3 × 1889 × 24/1000 = 5771 kWh

If the house had been located in Berlin, instead, which has 2600 degree days, then the heating load would have been much higher:

• Annual consumption = 127.3 × 2600 × 24/1000 = 7944 kWh

Put another way, it would have to be better insulated to achieve the same heating demand.

We can go further and say that if we managed to trim 1 W K^–1 off the heat loss coefficient by better insulation or airtightness, then the marginal saving in space heating demand would be 1889 × 24 = 45.3 kWh in London or 2600 × 24 = 62.4 kWh in Berlin. This could then be used to analyse the relative cost effectiveness of further energy-saving investments.