Seasonal Heating Energy (Degree-Day Method)

The degree-day method assumes a building's heat loss is proportional to how much colder it is outside, which is very nearly true, so seasonal energy is just the design load scaled by the ratio of average deficit to design deficit, stretched over the season and divided by the equipment's efficiency. Heating degree-days are exactly this quantity in disguise: HDD is the sum of (base temperature − daily mean) over the season, so ΔT_m = HDD ÷ days. A location with 5,000 °F-days over a 200-day season averages a 25 °F deficit.

Worked example: a house losing 60,000 BTU/hr at a 70 °F design difference, in that 5,000-HDD climate, with an 80 % AFUE furnace, burns 60,000 × (25/70) × 4,800 hr ÷ 0.80 ≈ 129 million BTU, about 1,290 therms of gas. The method dates to the 1930s, when American gas utilities used it to forecast next winter's demand, and it still underpins weather-normalised utility bill analysis. Its limits are worth knowing: the traditional 65 °F base assumes internal gains cover the first few degrees, which is wrong for a modern tight house (where 60 °F or lower is a better base) and wrong for a data centre (which needs cooling in January). It also ignores solar gain, wind and thermostat setback, so treat the answer as ±15 %, not as a bill.