Heating & Cooling Degree Days
Compute heating degree days (HDD) and cooling degree days (CDD) from a list of daily mean temperatures, free online.
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HDD for a day = max(0, base − mean temperature); CDD = max(0, mean − base). Sum over the period to estimate heating or cooling demand — fuel use scales roughly with HDD. Typical base temperatures: 18 °C / 65 °F (US), 15.5 °C (UK).