The BT47 index measures the evolution of production costs for building electricity companies. Its composition includes labor, materials, and electrical equipment. We regularly observe confusion between this sector index and the energy price charged to the end customer, which skews contractual decisions from the estimation phase.
Methodological Break in 2023 and Consequences for Indexation Formulas
Since the publication of the April 2023 index, Insee has applied new weighting coefficients and new components to the BT47. The series has been linked to the value of March 2023 without retroactive revision. Any indexation formula that compares a value prior to March 2023 with a subsequent value mixes two distinct methodologies.
In practice, a contract signed in 2021 that refers to the BT47 for its revision clause without specifying the calculation base produces an arithmetically correct but economically biased result. We recommend including an explicit mention of the connection date and the methodological status of the series used in each amendment.
To understand the BT47 index from Insee in this new framework, it is essential to distinguish between values published before and after April 2023 and treat the junction as an accounting break, not just a simple update.
BT47 and Electricity Cost: Distinguishing Production Index from Energy Tariff
The BT47 is not an indicator of the price of electricity sold by an energy supplier. It reflects the costs incurred by the company performing the electricity lot on a construction site: cables, equipment, panels, hours of skilled labor. A manager who uses the BT47 to anticipate their energy bill is making a scope error.
This distinction has direct effects on budget management. When the price of copper rises, the BT47 reacts with a delay related to Insee’s monthly publication cycle. In contrast, an increase in the regulated electricity sale tariff has no mechanical impact on this index.
To manage project costs, it is therefore necessary to cross-reference the BT47 with other references: the BT01 index (all trades), the TP indexes for external networks, and the spot price of energy if the project owner directly bears the construction site’s consumption.
Provisional and Definitive Values: A Trap in Price Revision
Insee publishes the BT indexes according to a monthly schedule, but the first values released may be provisional. A company that invoices a work situation based on a provisional index risks having to issue a credit note if the definitive value is lower.
We observe that the majority of revision clauses do not specify which status of the value should be retained. The typical public contract simply refers to “the BT47 index published by Insee on the date of…” without distinguishing between provisional and definitive. Three points deserve to be locked in the contract:
- The reference date (date of price establishment, month of service order, or month M-3 of the situation)
- The required status of the index: accepted provisional or imposed definitive, with a regularization period in case of subsequent revision
- The official source for consultation on the Insee website and the obligation to keep a dated copy for each situation
Difference Between BT47 and Other BT Indexes: Choosing the Right Reference for the Electricity Lot
The BT47 can progress at a very different rate from other building indexes. The share of materials in its weighting (copper cables, equipment, conduits) makes it more sensitive to non-ferrous metal prices than the BT01, which is dominated by structural work.
A project owner who applies the BT01 as the sole index for all lots underestimates or overestimates the actual drift of the electricity lot. The table below summarizes the differences in scope:
| Index | Main Scope | Dominant Sensitivity |
|---|---|---|
| BT01 | All trades combined | Cement, steel, structural labor |
| BT47 | Building electricity | Copper, equipment, electrician labor |
| BT48 | Plumbing, sanitary, heating | Steel/copper pipes, HVAC equipment |
Using a neighboring index as a substitute for the BT47 produces a revision that is less representative of the concerned lot. On a construction site where the electricity lot represents a significant share of the total amount, the cumulative difference between BT01 and BT47 can significantly alter the net margin of the contracting company.
Integrating BT47 into a Contractual Cost Control Strategy
Price revision is not the only use of the BT47. This index is also used for updating prices between the date of the estimate and the start date of execution. Updating and revision are two legally distinct mechanisms that can coexist in the same contract.
Updating occurs before the start of work if a significant delay elapses between the submission of the offer and the service order. Revision applies during execution, situation by situation. A well-drafted contract provides for both, each with its own formula and reference date.
- Ensure that the revision formula uses the BT47 and not a composite index for the electricity lot
- Set the initial value date (month zero) to the month of price establishment, not the month of notification
- Include a safeguard clause if Insee modifies the composition of the index during the contract, as occurred in 2023
- Systematically archive the publication date and status (provisional or definitive) of each value used
A quarterly follow-up of the BT47, cross-referenced with the execution schedule, allows for anticipating the financial impact of upcoming situations and adjusting the cash flow plan before the gap becomes critical.
