Set your parameters before calculating degree days: period, location, and base temperature
Choose the calculation period (monthly, heating season, full year) based on your use case
Before starting a degree-day calculation, lock in a fixed period. For performance monitoring, a monthly step helps you reset drift. For heating sizing, work instead on the site’s heating season, in line with your readings (start-up, shutdown, intermittent operation). At portfolio or comparative scale, a full year avoids segmentation bias.
Set the base temperature (Tb) for your building and system (heating, possibly cooling)
| Use | Tb (°C) | When to use it |
|---|---|---|
| Heating | 18 | Common reference for degree days and energy balances |
| Cooling | 26 | If you are tracking cooling demand |
Keep a consistent Tb with your setpoint assumption and internal gains. Record Tb and the period in the calculation note attached to the file.
Link the project to a relevant weather station (distance, altitude, valley/plateau effect)
Choose a relevant and traceable station. Prioritize proximity, then altitude and exposure. In valley or plateau settings, compare two stations and keep the one that best matches the temperatures measured on site, especially the outdoor base temperature used in your assumptions.
Apply the degree-day formula correctly: from the day to the cumulative total
Calculate daily degree days: DJU = max(0 ; Tb − Tmoy)
Degree-day calculation happens at the daily level. First set Tb (base temperature) for the building or weather zone, then calculate the day’s average outdoor temperature (Tmoy). If Tb − Tmoy is negative, the degree-day value is 0. Do not mix heating and cooling. The sign convention must remain identical throughout your file (sizing, supporting documents, calculation note).
Calculate Tmoy correctly from your data (Tmin/Tmax, hourly interval)
| Available data | Tmoy calculation (°C) | Note |
|---|---|---|
| Hourly measurements | Arithmetic mean of the values | The most robust approach |
| Tmin and Tmax | (Tmin + Tmax) / 2 | Acceptable approximation |
With an irregular hourly interval, switch to a duration-weighted mean. Do not recalculate Tmoy from a monthly average; you would lose the “max(0)” logic.
Sum by month or season and handle missing days (rounding, interpolation)
Add the degree days day by day, then group them by month or heating season. Keep one decimal place and round only at the end. If days are missing, first fill them by recalculating from the raw series. Otherwise, interpolate Tmoy for a single isolated day. Beyond that, switch to a nearby station and document the rule applied.
Retrieve the right weather data for unified degree-day calculation in 2026
Identify the source of the readings: Météo-France stations, local networks, historical records
For a defensible degree-day calculation in a project file, start from a Météo-France station (ID, coordinates, altitude) that is the most representative of the project area. Alternatively, a local network (municipality, airport, operator) is acceptable if you document calibration, maintenance, and series continuity. Keep the extraction URL and the download date.
Choose the right data type: observations, normals, or reference weather year depending on the need
For operational monitoring and before/after comparison, use daily observations (mean, min, max temperature). For an estimate of “average” performance, rely on climatological normals (1991–2020 reference when available). A reference weather year is mainly used for simulations and helps justify the choice of period and station.
Import and prepare the series (CSV, spreadsheet) while keeping traceability for your files
- Standardize the date (ISO), time zone, and unit (°C).
- Check for gaps, duplicates, and outliers, then document the corrections.
- Keep the source file, the prepared file, and a timestamped processing log.
Check your results: consistency, basic conversions, and common pitfalls
Verify orders of magnitude and compare several years (climate variability, 2026 case)
At the order-of-magnitude level, a degree-day calculation is first checked for internal consistency. Verify that the sum of monthly degree days matches the annual total over the same heating period. Then compare 3 to 5 years, keeping in mind that an atypical 2026 season can shift the cumulative total without any calculation error. To settle the matter, compare with a nearby station at a similar altitude.
Avoid classic mistakes: wrong Tb, station too far away, inconsistent units and cumulative totals
- Tb set incorrectly (18, 19, or something else). A different value is enough to distort a comparison.
- Weather station too far away, or altitude/exposure offset that flattens the differences.
- Mixed units. °C.day and K.day are equivalent, but the time step (day, month, season) is not.
- Cumulative total over a rolling period (12 months) when the study expects a heating season.
Recalculate using another base temperature (conversion) to compare studies
To compare two studies, avoid a simple rule-of-three. The robust conversion is a recalculation from daily temperatures. Formula: DJU(Tb) = Σ max(0, Tb - Tmoy,j). You change Tb, rerun the sum, and document the temperature source.
If you want to go back to the fundamentals to interpret these checks correctly (annual cumulative total, station, heating season), see our guide on unified degree days (DJU).
| Data | Symbol | Unit | What to document |
|---|---|---|---|
| Base temperature | Tb | °C | Selected value and justification |
| Daily mean temperature | Tmoy,j | °C | Station, distance, altitude, period |
| Daily degree days | DJUj | °C.day | Calculation method and rounding |
Use degree-day calculations on site: sizing, monitoring, and justification
Adjust heating consumption with degree days to compare before/after works
For clean tracking, start from a degree-day calculation using the same base temperature and the same station. Then normalize heating consumption to a reference weather year. Simple normalization formula: Ecorr = Emeasured × DJUref / DJUperiod. You can then compare “constant winter” kWh values, which is useful for substantiating a works plan or diagnosing a control drift.
| Parameter | Unit | Use |
|---|---|---|
| Base temperature Tb | °C | Keep the same before and after |
| Period degree days | °C.day | Actual weather over the period |
| Reference degree days | °C.day | Average year, same source |
Help with preliminary sizing (heat pump, boiler, radiators) using degree days and observed needs
Degree days give you a seasonal slope. From measured heating kWh, estimate G = E / (24 × DJU), in kW/°C. You obtain an approximate peak power: P ≈ G × (Tb - Text). This is preliminary sizing. You then confirm it with room-by-room heat losses and the local base temperature. For an air-to-water heat pump, you can cross-check with the sizing tool.
Document your choices: calculation note, weather evidence, and use in applications (CEE, MaPrimeRénov’)
In the note, freeze the degree-day source (provider, station, or grid), Tb, periods, energy considered, and assumptions (occupancy, setpoints, backup heating). Archive the weather evidence (export, timestamped screenshot) and the spreadsheet. For CEE or MaPrimeRénov’, this package mainly serves to justify sizing and monitoring, should there be a question during inspection or after-sales support.
