On the same mid-season day, two methods for calculating heating degree days give 2 and 2.88 degree days: a 44% gap on the same weather. Yet this figure is used to compare two heating winters and to measure how severe a season's weather was. It also sets the weather-corrected consumption that is compared with an energy baseline and its reference year. Before you sign off a weather correction, remember this: degree days correct for the weather, they do not correct for how the building is operated.
What is a degree day: definition, base 18 and heating or cooling degree days
The heating degree day, a measure of how cold a day was
A heating degree day (HDD, called DJU, degré jour unifié, in France) is the gap between a day's mean outdoor temperature and a reference temperature of 18 °C. It reflects how severe the weather was that day: the colder it is, the more heating the building needs.
Heating degree days (HDD) are calculated from the data of a weather station. The indicator applies whatever the energy source: natural gas, district heating or heat pump. It is not the same thing as the regulatory climate zone, which is fixed once and for all. It is also used to verify savings under the IPMVP, the International Performance Measurement and Verification Protocol.
Did you know? Base 18 is not a comfort setpoint. It starts from an indoor setpoint of 19 °C, minus about 1 °C of free gains: occupants, equipment and solar radiation[1][1]Studies and reportsWikipedia (French), “Degré jour unifié” (heating degree day)See in sources ↓.
How do you calculate heating degree days and where do you find the data?
The weather method: mean of Tmin and Tmax compared with 18 °C
The so-called “weather” method, used by Météo-France (France's national weather service)[2][2]Technical documentationMétéo-France, “Fiche méthode Degrés Jours” (degree-day method sheet), formulas of the “weather” and “energy professionals” methods, as reproduced by Wikipedia (French), “Degré jour unifié”See in sources ↓, remains the simplest way to calculate heating degree days: HDD = 18 - (Tmin + Tmax) / 2, where Tmin and Tmax are the day's minimum and maximum temperatures. Example: a day with a mean of 5 °C gives 13 HDD. A day warmer than the base counts as 0. Adding them up per month, then per year over the heating season, gives the year's heating degree days.
Same weather, two methods: how many degree days?
Set the day's weather and the base temperature. Both methods apply to the same day.Degree-day sources: Météo-France, SDES, Cegibat and choice of weather station
Météo-France degree days are available station by station. The SDES (the statistics service of the French ministry for ecological transition) publishes a climate severity index at national, regional and département level, calculated on a 17 °C threshold rather than 18 °C[4][4]Studies and reportsSDES, “Indice de rigueur climatique” (climate severity index), weather correction of energy consumption statisticsSee in sources ↓. The Cegibat simulator from GRDF (France's main gas distribution operator) covers more than 500 stations, updated monthly[5][5]Technical documentationCegibat (GRDF), “Simulateur DJU” (degree-day simulator), more than 500 stationsSee in sources ↓. The decisive point remains the choice of weather station: the one closest to the site's municipality, kept the same from one year to the next.
What heating degree days are for: weather-correcting energy consumption
Comparing two winters: the weather-corrected consumption formula
Corrected weather-sensitive consumption = measured weather-sensitive consumption × reference HDD / HDD of the period
Weather-corrected consumption brings every winter back to the same weather severity. The correction applies only to the weather-sensitive share, the part that depends on heating demand. Domestic hot water has no place in it.
The kWh/HDD ratio is used in the same way to compare two heating seasons, in commercial buildings as in industry. The SDES applies this weather correction to its annual energy statistics[4][4]Studies and reportsSDES, “Indice de rigueur climatique” (climate severity index), weather correction of energy consumption statisticsSee in sources ↓. The adjustment specific to performance contracts is dealt with separately, in the next section.
What the line explains and what it lets through
Building a baseline for an energy performance contract
Baseline and reference year: over what period?
The baseline describes what the building consumes before any action is taken. In an energy performance contract (EPC), it takes the form of an energy baseline. It brings together consumption, heating degree days, occupancy and operating hours over a period covering at least one full usage cycle.
Several years are better than one. However, a baseline that is too old gets compared with winters already changed by climate change. The EPC reference year is therefore written into the contract, together with its justification.
Adjust to the conditions of the reference year or of the current year?
Two approaches to degree-day weather adjustment coexist. The most common brings the baseline to the conditions of the reporting period: it states what the building would have consumed without the actions. The other normalizes both periods to a climate year fixed in advance. The IPMVP distinguishes between the two[6][6]Technical documentationEVO, “International Performance Measurement and Verification Protocol (IPMVP)”, routine and non-routine adjustmentsSee in sources ↓.
| Approach | Weather conditions used | What the figure tells you | Watch point |
|---|---|---|---|
| Baseline adjusted to the reporting period | those of the measured year | state what the building would have consumed without the actions | the savings in kWh vary with each winter |
| Normalization to fixed conditions | a climate year fixed in advance | compare years with each other | the chosen climate year must be written into the contract |
After a mild winter, the savings shown in kWh change depending on the approach chosen, even with the same performance in kWh/HDD. Insist that it be named in the annual report.
Occupancy, floor area, use: the other adjustment variables
Heating degree days are not the only variable. Occupancy rate, heated floor area, operating hours and change of use also affect a site's consumption. Routine adjustments are built into the model, with one indicator per variable. Non-routine adjustments, such as a floor being vacated or an extension, are justified case by case[6][6]Technical documentationEVO, “International Performance Measurement and Verification Protocol (IPMVP)”, routine and non-routine adjustmentsSee in sources ↓. Everything is written into the measurement and verification plan, the M&V plan.
An adjustment missing from the plan becomes a negotiation.
A wrong baseline skews the whole EPC, savings after savings
A wrong baseline carries over into every annual report, until the end of the contract. Whoever produces the figure is often the one who will be paid on it. The building owner receives it with no way of recalculating it.
The savings sharing, calculated on this basis, inherits every approximation. A baseline error does not show in the first year, it costs you every year.
Six questions to ask before accepting degree-day-corrected savings
Can a third party verify your weather correction?
Tick an item only if the answer is written in the contract or the annual report.Before accepting “degree-day-corrected” savings, have these six questions documented by the O&M contractor, the EPC holder or the software vendor. Tick those whose answer is already in writing.
An empty box does not prove the savings are wrong. It signals that no third party can verify them as things stand, which should be settled before the annual report is signed.
What Foobot does with degree days on an existing BMS
Making meters reliable before building the baseline
A baseline is only as good as its meters. Our BMS advisory, where we act on the building owner's side on the building management system (BMS), starts by tracking down whatever distorts the history:
- meter disconnections that never raised an alarm
- gaps in the trend-logged data
- duplicate points in the supervision software
- invisible drifts and unexplained peaks
As long as these faults remain in the history, no weather correction will make up for them.
Before signing off a baseline, check what the degree days correct
Degree days do one thing only: neutralize the weather from one winter to the next. They say nothing about manual overrides, a setpoint raised after a VIP complaint or an AHU (air handling unit) running at night.
A baseline should therefore be checked before the first annual report, not after the third year of the contract. Method, station, base and period are written down in black and white. The rest depends on the quality of the historical data. The first step fits in one sentence: make reliable the meters that build the baseline.
make reliable the meters that build the baseline
Further reading:
FAQ - Heating degree days
How do you calculate heating degree days?
A day's heating degree days equal the base, often 18 °C, minus (Tmin + Tmax) / 2. A day warmer than the base counts as zero. Monthly heating degree days are the sum of these values; the calculator above compares the two methods.
What is the reference degree-day figure for a heating contractor, by climate zone?
There is no single degree-day figure per zone. From the owner's point of view, the reference depends on the station and the period chosen. The heating contractor and the building owner must read it in the contract, not infer it from a climate zone.
Why are degree days an essential tool for heating contractors?
Under an O&M contract, the contractor uses them to optimize boiler room settings and plan interventions. Degree days also link the contractor's results to how severe the winter was. The owner therefore checks the method written into the contract.
Need a tool to track your degree days and optimize your energy use?
The calculation tool is the easy part: Cegibat or the SDES provide the data[4][4]Studies and reportsSDES, “Indice de rigueur climatique” (climate severity index), weather correction of energy consumption statisticsSee in sources ↓[5][5]Technical documentationCegibat (GRDF), “Simulateur DJU” (degree-day simulator), more than 500 stationsSee in sources ↓. The difficulty lies elsewhere. Without reliable meters or a documented IPMVP model, monitoring produces figures nobody can verify.
What are the limits of degree days?
Degree days ignore the building's thermal inertia and actual variations in solar gains. Their fixed base differs from real setpoints. They only correct the weather-sensitive share, never the baseload. Climate change also makes older baselines less representative.
Where can you get degree-day data?
Météo-France publishes degree days by station. The SDES publishes a climate severity index at national, regional and département level[4][4]Studies and reportsSDES, “Indice de rigueur climatique” (climate severity index), weather correction of energy consumption statisticsSee in sources ↓. The Cegibat simulator covers more than 500 stations[5][5]Technical documentationCegibat (GRDF), “Simulateur DJU” (degree-day simulator), more than 500 stationsSee in sources ↓. Choose the station closest to the site.
Degree days and climate zone: what is the difference?
The regulatory climate zone is a fixed zoning: zones H1, H2 and H3 and their sub-zones, set by département in Annex I of the French order (arrêté) of 26 October 2010. Degree days, by contrast, measure the weather severity of each day. Two sites in the same zone can show very different totals.
Sources and references
6 sources.- [1]Wikipedia (French), “Degré jour unifié” (heating degree day)↑
- [3]Cerema Hauts-de-France, “Méthode d'utilisation des Degrés Jours Unifiés (DJU)…” (method for using heating degree days to weather-correct energy consumption), 2023
- [4]SDES, “Indice de rigueur climatique” (climate severity index), weather correction of energy consumption statistics↑ 1 2 3 4
- [2]Météo-France, “Fiche méthode Degrés Jours” (degree-day method sheet), formulas of the “weather” and “energy professionals” methods, as reproduced by Wikipedia (French), “Degré jour unifié”↑
- [5]Cegibat (GRDF), “Simulateur DJU” (degree-day simulator), more than 500 stations↑ 1 2 3
- [6]EVO, “International Performance Measurement and Verification Protocol (IPMVP)”, routine and non-routine adjustments↑ 1 2