degree-day

/D IH0 G R IY1 D EY0/
noun
  1. 1

    A measure of heating or cooling, computed as the integral of a function of time that generally varies with temperature.

  2. 2

    A unit used in estimating the fuel consumption for a building; equal to the number of degrees that the mean temperature, for a 24-hour day, is below the base temperature, often 65F.

Source: https://en.wiktionary.org/wiki/degree%20day, https://en.wiktionary.org/wiki/degree-day