HDD: measuring climate severity for your 2026 jobs

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

An HDD measures the gap between a base temperature and the average outdoor temperature. In France, the 18 °C base is common for heating, but nothing forces you to keep this value. Moving from 18 to 19 °C mechanically inflates HDD. Before comparing two sites or two years, check the base used.

Calculation methods: daily, monthly, annual

The most common formula is daily. HDD = max(0, Tbase - Tavg for the day). Tavg often comes from (Tmin + Tmax)/2. Monthly or annual figures add up the daily HDD. Some tools calculate monthly figures from monthly averages, with a slight discrepancy. Keep the same method from start to finish.

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

For a heat pump, HDD alone don't give you the power, but they help choose a coherent strategy. Colder zone, higher HDD. You secure the sizing by cross-referencing HDD, local base temperature, heating curve and peak need. Goal: cover most hours with a good COP, without oversizing.

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

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.