Heating Degree Days
Cumulative measure of how much and how long outdoor temperature sits below a base.
Formula
Variables
Example
Base 18°C, avg 10°C for 30 days: HDD = 240
How it works
Heating Degree Days (HDD) measure how strongly and for how long outdoor temperatures fall below a chosen indoor comfort or balance temperature. For each day, you subtract the day’s average outdoor temperature from the base temperature; only days below the base contribute, and larger gaps contribute more because the building needs more heating. Adding these daily deficits over a season gives a useful estimate of total heating demand and lets you compare different climates or years on the same scale.
When to use it
Use HDD when estimating seasonal heating energy use, comparing winter severity between locations, or normalizing fuel consumption against weather. It is common in HVAC design, energy analysis, and utility planning.
Worked example, step by step
Suppose the base temperature is 18°C and the daily average outdoor temperatures for 5 days are 16°C, 12°C, 18°C, 9°C, and 14°C. Day 1: 18 - 16 = 2 Day 2: 18 - 12 = 6 Day 3: 18 - 18 = 0 Day 4: 18 - 9 = 9 Day 5: 18 - 14 = 4 HDD = 2 + 6 + 0 + 9 + 4 = 21 degree-days
Common mistakes to avoid
- Including days that are above the base temperature as negative values instead of counting them as zero
- Using hourly temperatures directly without first matching the method to daily averages or a consistent time step
- Confusing the base temperature with a thermostat setting instead of a chosen reference temperature for heating demand
Did You Know?
Utilities use degree days to predict seasonal heating fuel demand with surprising accuracy.
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