You look at your DPE (Diagnostic de Performance Énergétique — the French Energy Performance Certificate, or EPC) and one question keeps coming back: where does this figure come from? Your home is rated E at 280 kWh/m²/year, but your bills tell a completely different story. The answer fits in a few words: the 3CL method. It is what turns your wall insulation, your boiler type and your town's climate zone into a letter from A to G. And, crucially, it is a conventional calculation — not a measurement of what you actually consume.
Understanding the 3CL method means you stop enduring a label and finally start reading it. It means knowing why two neighbouring homes can be rated differently, why your EPC does not match your meter readings, and where to act to gain a class. As the publisher of a 3CL calculation engine, here is that method taken apart piece by piece, with no needless jargon.
A calculation, not a measurement. The 3CL-DPE 2021 method (decree of 31 March 2021) estimates a conventional consumption from the building fabric and a standardised usage. That is why your EPC can differ markedly from your real bills — without being wrong for all that.
What this article covers
What the 3CL method is and why it is a conventional calculation, the five uses it accounts for, how it reconstructs your consumption from the building fabric and the climate zone, the difference between final energy and primary energy (the famous coefficient), why your EPC differs from your bills, and the strengths and limits of this single method used across France.
The 3CL method: the legal framework of a single calculation
Since 1 July 2021, every home EPC in France has been produced using a single calculation method, known as 3CL-DPE 2021, defined by the decree of 31 March 2021 on the methods and procedures applicable to the energy performance certificate and to the software producing it. The acronym "3CL" stands for Calcul de la Consommation Conventionnelle des Logements (Conventional Consumption Calculation for Dwellings) — the word "conventional" is the essential part.
This unification is a break with the past. Before 2021, several methods coexisted, including a "bill-based" one that reconstructed consumption from the occupants' real readings. That approach has now been abandoned for homes: the 3CL-DPE 2021 is mandatory for all, which guarantees that two identical homes get the same EPC, regardless of the habits of those living in them.
The decree of 31 March 2021 also made the EPC legally binding (CCH art. L.271-4): its information engages the seller's and landlord's liability, unlike the earlier EPC which was merely indicative. The direct consequence: the rigour of the calculation and the survey is no longer a technical detail but a legal issue. An EPC produced by the 3CL is also valid for ten years (CCH art. D.126-19, from decree no. 2021-872 of 30 June 2021).
| Element | What it is | Reference |
|---|---|---|
| 3CL-DPE 2021 method | Single conventional calculation for all homes | Decree of 31 March 2021 |
| Legally binding | The EPC engages the seller's and landlord's liability | CCH art. L.271-4 |
| Validity | 10 years (subject to transitional rules) | CCH art. D.126-19; decree no. 2021-872 |
| Electricity coefficient | 2.3 until 31/12/2025, then 1.9 | Order of 13 August 2025 |
How the 3CL method works: a calculation, not a measurement
Here is the heart of the matter, and the source of almost every misunderstanding. The 3CL method never reads your meters. It reconstructs your home's theoretical consumption from its physical characteristics, applying calculation conventions set by regulation. It is a model, in the scientific sense: a simplified but consistent representation of reality.
The input data: the building fabric and the equipment
The certified assessor visits the home and records its characteristics: habitable floor area, ceiling height, orientation, insulation of walls, roof and floors, type and performance of windows (single, double or triple glazing), heating system and its efficiency, hot-water production method, ventilation type and year of construction. These data describe the building's envelope and its equipment: the two main families of inputs to the calculation.
The accuracy of this survey is decisive. The 3CL method is rigorous, but it is only as good as its inputs: two assessors who interpret the insulation of an old wall or the age of a boiler differently can reach two different labels. The calculation itself is identical; it is the data entry that varies.
The climate zone and the standardised usage
To these home-specific data, the method adds conventions. The first is the climate zone: mainland France is divided into eight zones (H1a to H3) according to the harshness of winter and the mildness of summer. The same home rated D in Nice might be rated worse in Strasbourg, simply because the calculation assumes a colder winter there, hence a higher heating need.
The second convention is the standardised usage. The calculation assumes a reference set-point temperature (around 19°C during the day), a typical occupancy scenario and an average behaviour — not yours. That is precisely what makes EPCs comparable to one another: you assess the home, not the occupant. But it is also what explains the gap with your bills, as we will see below.
Calculating needs, then consumption
From these inputs, the method proceeds in two stages. It first calculates the home's needs: how much heat must be supplied to keep it at temperature, given its heat losses through walls, roof, windows and ventilation, and its free gains (sun, occupants, equipment). It then translates those needs into energy consumption, accounting for the actual efficiency of the equipment: an efficient boiler uses less for the same heat need. For a detailed reading of each section of the report, see our guide on how to read an EPC page by page.
The five uses accounted for by the 3CL method
The 3CL method does not calculate "all" of a home's consumption. It is limited to a precise regulatory perimeter: five uses, and only those. This is key to understanding the gap with your bills, which, by contrast, add up everything.
| Use | What it covers |
|---|---|
| 1. Heating | The main item: keeping the home at temperature in winter |
| 2. Domestic hot water (DHW) | Hot-water production for domestic use |
| 3. Cooling | Air conditioning, where a system is present |
| 4. Lighting | Conventional lighting consumption of the premises |
| 5. Auxiliaries | Ventilation, pumps and circulators linked to heating and DHW |
Heating and domestic hot water make up most of the result in the majority of homes. Cooling is counted only if air conditioning exists. Lighting and auxiliaries (the term covers the equipment that keeps the systems running: ventilation fans, circulation pumps) are smaller items, but are integrated by regulation.
Key takeaway: cooking and household appliances (fridge, washing machine, television, computers) are not part of the five uses. This is one reason your total electricity bill often exceeds the consumption shown on the EPC: it includes uses that the 3CL method deliberately ignores.
Final energy and primary energy: the role of the coefficient
This is one of the least understood notions of the EPC, yet it often decides the class. The 3CL method distinguishes two quantities: final energy and primary energy.
Final energy is what you buy and what passes through your meter: the kilowatt-hours of electricity or gas billed. Primary energy, by contrast, includes the upstream losses of production, transformation and transport of the energy, from the source to your home. To move from one to the other, the method applies a primary-energy conversion coefficient specific to each source.
For gas, heating oil or wood, this coefficient is 1: 1 kWh consumed counts as 1 kWh of primary energy. For electricity, the coefficient was set at 2.3 in the 3CL-DPE 2021 method: each kilowatt-hour of electricity consumed counted as 2.3 kWh of primary energy in the energy label. This factor, inherited from the historical efficiency of the power system, heavily penalised electrically heated homes.
Key takeaway: the EPC's energy label is expressed in primary energy (kWh PE/m²/year), not final energy. An electric home could therefore show a degraded energy label even though its real consumption, at the meter, was modest. That is the whole point of the coefficient.
This coefficient has now changed. The order of 13 August 2025 (published in the Official Journal on 26 August 2025) lowered it from 2.3 to 1.9 on 1 January 2026, to better reflect the largely decarbonised nature of the French electricity mix. Mechanically, for the same final consumption, the calculated primary energy of an electric home falls by around 17%, which can earn it a letter when it is close to a threshold. This reform illustrates a reality of the 3CL method: a label is not an immutable physical fact, but the product of a calculation and of conventions that evolve. To gauge the concrete scale of this reform, see our analysis of the 850,000 homes lifted out of classes F and G in 2026.
Why your EPC differs from your bills
This is the question every owner asks one day: "my EPC says 280 kWh/m²/year, but I use far less — who is wrong?" The answer: neither. The two measure different things, and the gap is expected by the very design of the method.
Three main reasons explain this gap. First, the standardised usage: if you heat to 18°C, close off rooms or are rarely home, you use less than the conventional scenario — and vice versa if you heat to 22°C. Second, the perimeter of the five uses: your bill includes cooking and appliances, which the EPC excludes. Third, primary energy: the energy label applies the conversion coefficient, whereas your bill is in final energy.
This gap can reach 50% or more, in either direction. A thrifty occupant will see their EPC "overstate" consumption; a large household that heats heavily will see the opposite. This is not a flaw in the certificate: it is the price of comparability. Without standardised usage, you would be rating the occupant rather than the home, and the same property would change class with every move.
Profile: couple with no children, working from home, heating to 19°C, often away. Their real consumption is well below the EPC's conventional scenario. For a future buyer with two children at home, it is the conventional label that will be relevant, not the sellers' current bill.
Strengths and limits of the 3CL method
The 3CL method is neither perfect nor absurd: it is a comparison tool, with real qualities and acknowledged limits. Knowing both sides avoids both rejecting it outright and making it say what it does not.
Its strengths: comparability and reproducibility
The method's main asset is comparability. Because it assesses the home independently of its occupants, it allows two properties to be compared on a consistent basis — exactly what a buyer or a tenant needs. It is also a reproducible method: with identical inputs, two thorough assessors get the same result, which makes it a foundation for binding decisions (rental bans, grants, green value).
Its limits: sensitivity to data and gap with reality
Its main limit is its sensitivity to the input data. On old building stock, where the real insulation of walls is hard to establish without probing, the method applies default values that can diverge from reality. A rushed survey or over-favourable assumptions produce a fragile label. This is precisely where detectable inconsistencies lie: an old, never-renovated building rated A, or a label very different from comparable homes in the same building, deserve a second look.
The second limit is the gap with reality already described: the method answers "what is this home's intrinsic performance?", not "how much will I pay?". Confusing the two leads to disappointment or poor trade-offs. The right reading is to see the EPC as a performance rating of the building fabric, to be complemented by a bill estimate tailored to your usage. To place the method within the overall certificate, see our guide on what an EPC is.
⚠️ Warning: a single method does not mean an infallible result. Consumer-press tests have shown that the same home can be rated differently depending on the assessor, for want of a consistent survey. In the event of an obvious inconsistency, a check is needed before signing.
Estimate your label with the 3CL method
Enter your home's floor area, insulation, heating energy and climate zone: the simulator applies the logic of the 3CL method to estimate your consumption across the five uses, your two labels (energy and climate) and your class from A to G — with the 1.9 electricity coefficient built in since 2026.
To go further: the complete EPC guide and the guide to EPC classes A to G.
Conclusion
The 3CL-DPE 2021 method (decree of 31 March 2021) is not a black box: it is a conventional calculation that turns the building fabric, the equipment and the climate zone into a comparable label, over the perimeter of the five uses. It does not measure your real consumption, and that is deliberate: it rates the home, not the occupant. Hence the gap with your bills, and the importance of the primary-energy coefficient — lowered from 2.3 to 1.9 for electricity on 1 January 2026.
Reading the 3CL method means knowing where to act to gain a class and spotting inconsistencies before signing. The OneDpe EPC simulator applies this calculation logic to your own characteristics: it estimates your five uses, your two labels and your class, 2026 coefficient included.
