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Seasonal Heating Energy (Degree-Day Method)

Also known as degree day method · annual heating estimate

E=Q˙d ΔTm tΔTd ηE = \frac{\dot{Q}_d \, \Delta T_m \, t}{\Delta T_d \, \eta}

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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 ΔTm = 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.

Seasonal Heating Energy (Degree-Day Method)
E=Q˙d ΔTm tΔTd ηE = \frac{\dot{Q}_d \, \Delta T_m \, t}{\Delta T_d \, \eta}
Where
  • EE= Seasonal energy input
  • Q˙d\dot{Q}_d= Design heat loss
  • ΔTm\Delta T_m= Average temperature deficit
  • ΔTd\Delta T_d= Design temperature difference
  • tt= Season length
  • η\eta= Seasonal efficiency
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