A unified degree day is a degree day computed for a base temperature of 18 °C. Its calculation method is officially recognised and splits into three cases depending on where those 18 °C fall between the day's minimum and maximum: zero if the minimum is above 18 °C, a plain gap from the mean if the maximum stays below 18 °C, and a dedicated formula in between. The reference series is published in ten-day blocks from 1 September to 30 June, for 102 French weather stations. The 18 °C is not a heating setpoint: the base assumes 2 to 3 °C come from solar and internal gains, for a typical indoor temperature of 20 to 21 °C.
HDD: a simple definition and a direct link to climate severity
What HDD are used for in building and energy retrofit
HDD, or "unified degree-days", express climate severity over a period. The colder (or hotter) it is compared to a reference temperature, the higher the HDD figure climbs. In building work, they're used as a simple reference point to compare consumption from one year to the next, or between two sites, by correcting for the weather effect.
Heating HDD vs. cooling HDD: choosing the right one for your works
Heating HDD measure the heating need when the outdoor temperature drops below a base (often 18 °C). Cooling HDD do the opposite, above a higher base, useful for sizing and tracking air conditioning or a reversible heat pump. Choose the indicator based on the item targeted. Insulation and airtightness mainly affect heating, solar protection and ventilation affect cooling.
What HDD don't tell you: limits to know on site
HDD don't see everything. They don't account for occupancy, actual setpoints, internal gains, wind, or local overheating. To assess a job, always cross-reference HDD, consumption readings, and feedback from occupants. Common sense before figures.
How to calculate and read HDD without making mistakes
Base temperature: the key point that changes the result
The unified label is reserved for the 18 °C base: a degree day computed on another base is a degree day, not a unified one. That is the whole point of the word, it guarantees two series can be compared. Nothing stops you working on another base for a particular purpose, but you then have to say so in the calculation note and stop calling the result a unified degree day. Moving from 18 to 19 °C mechanically inflates the figure, and a before-and-after comparison becomes unusable if the base shifted in between.
Calculation methods: daily, monthly, annual
The formula you meet everywhere, HDD = max(0, 18 − (Tmin + Tmax)/2), is exact in only one of the three official cases. Here are all three.
| Case | Condition | Formula |
|---|---|---|
| Summer | Tmin ≥ 18 °C | HDD = 0 |
| Winter | Tmax ≤ 18 °C | HDD = 18 − (Tmin + Tmax) / 2 |
| Mid-season | Tmin < 18 °C < Tmax | HDD = a × b × (0.08 + 0.42 × b) |
With a = Tmax − Tmin and b = (18 − Tmin) / (Tmax − Tmin).
The gap is not theoretical. Take an April day with a minimum of 14 °C and a maximum of 24 °C: the simplified formula gives a mean of 19 °C, so zero degree days, whereas the official method gives a = 10, b = 0.4 and HDD = 10 × 0.4 × (0.08 + 0.168) ≈ 1.0. On a day at 8 and 20 °C, the simplified version gives 4.0 against 4.3. The divergence therefore sits on mid-season days, the ones that fill the start and the end of the heating season.
A second, less known trap: the readings do not follow the calendar day. The minimum used is the one observed between 18:00 the previous day and 18:00 on the day in question, the maximum between 06:00 that day and 06:00 the next. Recomputing degree days from a midnight-to-midnight export does not reproduce the reference figures.
Comparing periods: common pitfalls and good practice
To correct a heating bill, people often talk about consumption "corrected for HDD". The right reflex is to make the data comparable, otherwise you're comparing apples and radiators.
- Same base, same weather station or climate zone, same time step.
- Compare full heating seasons, not a single month.
- Watch out for leap years and changes of sensor or measurement location.
Using HDD to estimate heating needs and justify your choices
Sizing insulation: linking HDD, heat losses and consumption
HDD (unified degree-days) express the "severity" of a climate. With your heat losses (in W/K), you can estimate an annual need. Order of magnitude: need (kWh) ≈ heat losses x HDD x 24 / 1000. You then test your insulation scenarios. If you lower heat losses by 30%, heating consumption follows the same slope. Simple and clear.
Choosing a heat pump: matching power to climate severity
Degree days do not size a heat pump, and that is a confusion worth holding firmly against a client or a supplier. A degree day describes a quantity of energy across a season, whereas output is calculated at the peak, from the local base temperature and the EN 12831 method. Two towns can post similar degree days and base temperatures several degrees apart. It is that method, not the degree day, that fills the heat loss report built from the survey readings. Degree days serve downstream: estimating the annual need, correcting a bill for the weather, and checking afterwards that the machine worked within the intended range.
Presenting the case to the client: talk comfort and bills, not theory
Show a "before, after" comparison over a typical year for the area. Explain that HDD vary from one town to another, so the bill does too, even for an identical home. Then translate that into benefits. More stable comfort, fewer start-ups, and an estimate of savings in euros.
HDD and performance tracking after works: checking your results in 2026
Normalising consumption: comparing before/after at equivalent climate
To measure the real effect of works, compare your kWh over a similar period while correcting for weather using HDD. In practice, you calculate a simple ratio: heating kWh divided by HDD. This avoids jumping to conclusions too fast after a milder or colder winter.
Spotting a setting or usage that skews everything (thermostat, schedules, hot water)
If the ratio doesn't move, look at usage. Setpoint too high, extended time slots, too aggressive a reheat, or hot water that consumes continuously. A check of the settings and the backup can be enough to recover real gains.
Setting up a simple tracking table for your jobs
Over 2026, keep a monthly table: meter reading, kWh, HDD for the month, kWh/HDD ratio, setpoint temperature, anomalies. With 10 minutes a month, you get a clear read and can adjust before the bill runs away.
Where to find reliable HDD and how to work them into your files
Data sources: weather stations, communes, climate zones
The order of magnitude worth knowing by heart, station by station, fits in one table. It serves as a plausibility check, not as evidence: a calculation note is justified with the COSTIC or Météo-France series for the station you selected, not with these rounded figures.
| Reference station | Climate zone | HDD base 18, heating season | Reading |
|---|---|---|---|
| Nice | H3 | 1,100 to 1,300 | national floor, short heating season |
| Marseille | H3 | 1,400 to 1,600 | mistral, marked day-night swings |
| Bordeaux | H2c | 1,800 to 2,000 | long mid-season, high share of mixed days |
| Nantes | H2b | 1,900 to 2,100 | oceanic, few extremes |
| Rennes | H2a | 1,950 to 2,150 | fully oceanic |
| Paris and Trappes | H1a | 2,300 to 2,500 | reference station for zone H1a |
| Lyon | H1c | 2,350 to 2,550 | attenuated continental, valley floor |
| Lille | H1a | 2,500 to 2,700 | moderate cold but a long season |
| Clermont-Ferrand | H1c | 2,450 to 2,650 | 330 m altitude, basin effect |
| Nancy | H1b | 2,600 to 2,800 | reference station for zone H1b |
| Strasbourg | H1b | 2,800 to 3,000 | metropolitan ceiling outside the mountains |
Two lessons for pricing. First, the ratio between the north-east and the Côte d'Azur is above 2.5, which rules out carrying a kWh per square metre figure from one job to the next without recalibration. Second, within one climate zone the gap between two stations stays under 10%, which makes the zone good enough for a first estimate and not good enough as soon as a performance commitment is signed.
For reliable HDD, start with Météo-France data. The ClimatHD tool gives series by station and climate normals. On data.gouv.fr, you'll find territorial extracts drawn from these stations. Climate zones (H1, H2, H3) then serve as a safeguard to check that the order of magnitude matches the area.
Choosing the right location: altitude, microclimate, coastal proximity
Avoid picking the commune "in the centre" by default. Compare the nearest station while accounting for altitude, enclosed valleys and coastal effects. In the mountains or by the sea, two neighbouring villages can show very different HDD. If in doubt, keep both values and justify the one you use.
Archiving your evidence: keeping HDD in your reports and quotes
In your files, attach a dated export (screenshot or file) with the period, the calculation base (e.g. 18 °C) and the source. Reuse the value in the audit, the heat pump sizing and the quote, with an HDD evidence callout. Also keep the dataset identifier to retrieve the history.
