DJU (Unified Degree Days)

Heating degree days (HDD) measure, over a given period, the cumulative difference between the outside temperature and a chosen base temperature. They are used to quantify the severity of a winter (heating HDD) or a summer (cooling HDD), to normalize energy consumption, and to compare years/sites. In practice, the higher the heating HDD, the colder the season and the greater the heating requirements; the higher the cooling HDD, the hotter the summer and the greater the air conditioning requirements.

DJU: operational definition and calculation

For a heating base temperature (often 18°C ​​in France), the following is calculated daily:

- DJU₍heating₎(day) = max(0, Tbase − Tmoy.ext), where Tmoy.ext is the average (Tmin+Tmax)/2.

We then accumulate this over the month or year. Quick example: Base temperature 18°C, Average temperature 8°C → 10 heating degree days (HDD) for the day.

- For cold, it's the opposite : DJU₍cold₎(day) = max(0, Tmoy.ext − Tbase_cold), with a base often 22–24 °C depending on the use.

Choosing the baseline depends on the setpoint and the building's thermal inertia (18°C ≠ 20°C). Heating degree days (HDD) are available for each weather station and are broken down monthly/annually. They allow for the creation of correlation curves between kWh and HDD (regressions) to isolate the weather-related contribution to energy consumption, estimate a reference consumption under "normal weather conditions," or compare two sites/climates.

DJU in planning and operation: advantages, limitations and points to consider

Interests

• Normalizing consumption: comparing a harsh winter to a mild winter.
• Monitor performance: if kWh/DJU decreases, operation improves (adjustments, balancing, water law).
• Forecast: estimate heating/cooling kWh according to weather scenarios (budget).
• Multi-site benchmarking: bringing consumption down to a common indicator.

Boundaries

• Assumption of linearity of needs ↔ DJU (reality impacted by solar inputs, wind, occupancy).
• Base temperature sometimes unsuitable for the building (over/underestimation).
• It does not deal with fine summer comfort: the RE2020 rather uses DH (degree-hours) for overheating.

Points to consider

• Consistent basis: choose 18 °C (or 20 °C) for heating according to setpoint/inertia; 22–24 °C for cooling according to use.
• Single source: using the same station and the same method from one month to the next.
• Scope: clean the kWh of non-climate uses (process, office automation) before correlation.
• Analysis: follow kWh/DJU and slope/intercept of the regression (the interception reveals the non-weather basis).
• Clarification: if kWh/DJU drifts, check water law, balancing, insulation, BMS schedules.
• Summer: Cold heating degree days are useful for air conditioning, but supplemented by heating factor, humidity and solar gains.

Anecdote — “DJU and invoice submitted correctly in Besançon”

In Besançon, an office building saw its energy bills rise despite a predicted "average" winter. A comparison of kWh versus 18-day heating degree days (HDD) revealed a steeper slope than the previous year and an increase in intercepted consumption (consumption excluding weather-related factors). An audit revealed an excessively steep heating curve, inadequate insulation in the ductwork, and "temporarily" extended operating hours for the building management system (BMS). By reducing the slope, re-insulating the risers, and tightening the operating hours, the kWh/HDD ratio returned to the previous year's level, confirming real energy savings independent of the weather.