Heating degree days calculator
Degree days measure how cold a period was relative to a base temperature, which is the honest way to compare one heating season with another.
W/K
%
per kWh
Degree days
1020.0K day
- Heat demand
- 6120 kWh
- Fuel energy
- 6955 kWh
- Heating cost
- 486.82
- Average daily fuel
- 57.95 kWh
- Degree days per day
- 8.50 K
- Heat demand at that temperature
- 2125
Show the working
- Mean outside temperature = 7
- Base temperature = 15.5
- Number of days = 120
- Building heat loss coefficient = 250
- Heating efficiency = 88
- Fuel price = 0.07
- Degree days per day = max(15.5 - 7, 0) = 8.5
- Degree days = 8.5 * 120 = 1020
- Heat demand = 250 * 1020 * 24 / 1000 = 6120
- Fuel energy = 6120 / (88/100) = 6954.55
- Heating cost = 6954.55 * 0.07 = 486.818
- Average daily fuel = 120 > 0 ? 6954.55 / 120 : 0 = 57.9545
- Heat demand at that temperature = 250 * (15.5 - 7) = 2125
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- degree days = (base temperature - mean temperature) x days, floored at zero
- heat demand = heat loss coefficient x degree days x 24 hours
The 15.5 degree base assumes internal gains from people, cooking and appliances lift the house the last few degrees. A well-insulated house has a lower base.
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