The degree-hour is the degree day brought down to the hour: DH = HDD × 24. A season at 2,900 degree days, the order of magnitude for the coldest French zone, therefore comes to roughly 70,000 °C·h, against 30,000 for a Mediterranean town. The full calculation chain fits in two lines, the building's loss coefficient G in W/K, then the annual need Q = G × DH / 1000 in kWh. The base stays the one used for unified degree days, 18 °C, and changing that base invalidates any comparison against a reference series.
Understanding degree-hours and their direct link to the EPC
Degree-day, degree-hour: simple definitions and useful differences on site
The heating degree day (HDD) adds up, over a day, the gap between a base temperature (often 18 °C) and the average outdoor temperature when it's colder. The degree-hour does the same thing, but hourly. It's a finer reference point. On site, it helps explain why two "cold" weeks don't look alike, and to discuss comfort, settings and power.
Why degree days and degree-hours influence the heating consumption estimated on an EPC
An EPC doesn't use your bills. It relies on a standard climate tied to the location. The colder the reference year, the higher the degree days or degree-hours climb. With identical insulation and system, the calculated heating need rises. For more on this, see heating degree days.
What the EPC actually "sees": local climate, thermal mass, surface area, usage (and what it doesn't replace)
The EPC "sees" the surface area, the envelope, thermal bridges, ventilation, equipment efficiency, and standard occupancy (temperature, schedules). It also factors in the building's thermal mass. However, it doesn't replace an on-site measurement, actual usage, or the microclimates that can differ from one street to the next.
Method for calculating degree-hours to estimate your heating needs
Data to gather: base temperature, heating period, HDD/degree-hours for your zone
Start from published degree days on the 18 °C unified base, over the 1 September to 30 June reference period, and convert with DH = HDD × 24. Recomputing degree days yourself is a false economy: the official method splits into three cases and uses readings that do not follow the calendar day, as set out in the article on unified degree days.
| Zone | Order-of-magnitude degree days | Degree-hours |
|---|---|---|
| Coldest, north-east | ~2,900 | ~70,000 °C·h |
| Temperate, northern and western basins | ~2,400 | ~58,000 °C·h |
| Mildest, Mediterranean coast | ~1,200 | ~29,000 °C·h |
These orders of magnitude are there to frame a costing, not to replace the figure from the nearest station. The gap between coldest and mildest is a factor of 2.4: for an identical dwelling, that is directly the ratio of heating consumptions.
The step-by-step formula: from degree-hours to heating needs (kWh) based on the envelope
1) Assess the heat loss coefficient G in W/K. Add up fabric losses, i.e. U × A, thermal bridges, i.e. ψ × L, and air renewal losses, i.e. 0.34 × Qv. 2) Calculate your degree-hours DH over the period. 3) Theoretical heating need in kWh. Q = G × DH / 1000. 4) For energy consumed, divide by the system's seasonal efficiency: see seasonal efficiency of generators.
Two errors derail this chain. The first is confusing G with D: G is a loss coefficient in W/K, independent of climate, whereas D is an output in W at the base temperature. You move from one to the other with D = G × (Tint − Tbase), and it is D, not G, that feeds machine sizing under the EN 12831 method. The second is forgetting free gains: the 18 °C base already includes them, to the tune of 2 to 3 °C, so counting them separately deducts them twice.
Common mistakes to avoid: unrealistic setpoints, thermal bridges, ventilation, solar gains
- Assuming a setpoint of 21 to 22 °C everywhere. Reality is often more nuanced from room to room.
- Forgetting thermal bridges. They inflate G, especially in partial retrofits.
- Underestimating ventilation and infiltration. Mechanical ventilation, air inlets, airtightness defects change everything.
- Ignoring solar and internal gains. They reduce needs, especially with large windows.
Using the EPC to prioritise heating and insulation works
Reading the EPC indicators that matter for your costing: band, consumption, emissions, heat losses
On an EPC, four pieces of information save time when quoting. The energy efficiency rating and the environmental impact rating give the target. Consumption in kWh/m²/year and emissions in kgCO2/m²/year are used to compare before and after works. And the breakdown of heat losses (roof, walls, floor, windows, ventilation) shows where the energy is escaping.
Linking degree-hours and heat losses: quickly spotting the items that "drive" heating
The degree-hour, close to degree-days, indicates the "length" of the heating season for your area. Cross-reference it with the EPC's heat losses. If the surfaces (walls, loft) dominate, insulation quickly lowers the needs. If ventilation and infiltration weigh heavily, target airtightness and suitable ventilation. You spot what's driving heating at a glance.
Choosing the right scenario: insulation, controls, generator replacement (heat pump, boiler, etc.)
- Address the envelope first on the main items.
- Add controls (programmable thermostat, thermostatic valves, balancing).
- Then size the generator (air-to-water heat pump, condensing boiler, biomass) for proper sizing after insulation.
On-site case: turning a calculation into a coherent quote (without losing a whole day)
House example: estimating the heating need before/after works from the degree-hour figure
For a 110 m² house, you gather the degree-hours for your commune (base 18 °C). You estimate the overall heat loss coefficient UA (in W/K) from the surfaces and ventilation. Annual need (kWh) = UA × DH / 1,000. Order-of-magnitude example with 50,000 K.h. Before works UA = 220, i.e. 11,000 kWh. After insulation and window/door work UA = 120, i.e. 6,000 kWh. This drop helps you size a generator or a heat pump without overselling.
How to justify your assumptions in the quote: transparency on temperatures, usage and limits
In the quote, list 3 or 4 key assumptions. Indoor temperature used (often 19 °C), occupancy periods, ventilation, reference weather, and the energy price used. Add the limits. This calculation is simplified, and doesn't replace either a thermal study or a formal assessment.
Presenting a readable gain to the client: savings, comfort, and consistency with the EPC
Show a before/after in kWh, in pounds, and in comfort. Fewer cold surfaces, a more stable temperature, and a system that runs less. Link it to the expected EPC, staying cautious. The client sees a clear gain, and you keep a consistent message about the possible variations depending on weather and usage.
Saving time with Argile: degree-hour, EPC and renovation scenarios in practice
Fast energy diagnosis: compare scenarios in under 5 minutes from the home's data
You enter the address, the surface area, the heating system and, if you have it, the existing EPC. Argile cross-references this information with public building records and climate data to reconstruct the home and estimate the needs, including a degree-hour reference point for summer comfort. In under 5 min, you compare 2 or 3 scenarios (insulation, ventilation, heat pump) and their impacts.
Feasibility analysis: detecting technical constraints (Open Data) that impact heating and performance
Before the visit, Argile surfaces the key constraints via Open Data. Protected area, local rules, hazards (flooding, clay shrink-swell), altitude and gas or electricity network. You avoid unrealistic quotes and choose the right heating solution faster.
Pre-costing and quote: integrating the funding and structuring a whole-house retrofit offer
Argile prepares a pre-costing and a quote with the funding built in. What the household qualifies for is taken into account to structure a whole-house retrofit offer, with a readable figure for what is left to pay and coherent packages. Enough to move fast, without selling blind.
