In the US, heating energy loads are calculated on degree-days.

This captures the expected dT on days when heating is required and ignores days when heating is not used. In the summer for example...

...except in the US cooling is also common so it actually captures both heating and cooling and involves multiple energy sources in many locations...which brings up that degree-days are highly variable at the local level -- consider the difference between Norway and Greece in the EU.

degree-day https://www.eia.gov/energyexplained/units-and-calculators/de...