Degree-days compress a whole stretch of weather into one number that heating and cooling energy scale with. Pick a base (balance) temperature — the outdoor temperature at which the building needs no heating, commonly 18 °C / 65 °F or 15.5 °C, lower for well-insulated buildings with high internal gains — then sum the daily shortfalls: HDD = Σ max(0, Tbase − Tmean) over the days, and CDD = Σ max(0, Tmean − Tbase) for the cooling side. A single day feeds only one of the two tallies, or neither if it sits exactly at the base. A 30-day month averaging 10 °C against an 18 °C base contributes 8 degrees a day, HDD = 240 K·day.
The tally becomes an energy estimate through the building’s heat-loss coefficient: Energy (kWh) = DD × 24 × UA / 1000 ÷ efficiency, where the efficiency slot takes a boiler fraction below 1 or a heat-pump COP above it. For the 240 K·day month, a building with UA = 200 W/K and a 90% boiler needs 240 × 24 × 200 / 1000 / 0.9 = 1,280 kWh — and at 0.15 per kWh, about 192 in fuel.
The method’s honest place is screening: it assumes a constant UA and ignores solar gains, intermittent operation, and dynamic effects, so it suits comparing periods, monitoring and targeting consumption, and rough sizing rather than replacing hourly simulation. The framework is standardized — the ASHRAE Handbook degree-day method, CIBSE TM41, and ISO 15927-6 for deriving degree-day data from temperature records.