Monthly utilisation factors are the basis of many procedures for calculation of monthly heating or cooling requirements for buildings, notably in the procedure described in standard ISO 13790:2008 ‘Energy performance of buildings – Calculation of energy use for space heating and cooling’, which is widely used for the implementation of the Energy Performance of Buildings Directive in Europe. The procedures used to determine the values of the factors are invariably empirical rather than being derived from first principles, with the principal parameter being the ratio between monthly mean heat gains and monthly mean heat losses for the space in question. This article shows that this ratio is inherently insufficient to define the values and illustrates how months with similar values of the ratio can have different utilisation factors. It also shows that, if daily heating requirement is proportional to outdoor temperature, the key building parameter needed to determine the utilisation factor is the familiar base temperature. The base temperature can be expressed in terms of the monthly gain: loss ratio and the mean indoor and external temperatures: the day-to-day frequency distributions of outdoor temperature is also important. Finally, the article demonstrates that, for many situations, the ISO 13790 procedure and a linear model with residuals produce similar estimates of monthly heating requirement. However, this is not true towards the upper end of its observed range. In this situation, the linear model produces lower values for utilisation factors and correspondingly higher heating (and cooling) requirements. This effect is most marked when the mean indoor and outdoor temperatures are close or the space is well-insulated (causing a given heat gain to represent a higher potential temperature difference).

Practical application: Monthly utilisation factors are the basis of many procedures for the calculation of monthly heating or cooling requirements for buildings, notably in the procedure described in standard ISO 13790:2008 ‘Energy performance of buildings – Calculation of energy use for space heating and cooling’, which is widely used for the implementation of the Energy Performance of Buildings Directive in Europe. This article shows that an alternative approach based on the concept of energy signatures, although producing very similar results in many situations, is a more robust and extendable basis for monthly heating and cooling energy demand calculations.