On an electrical installation site that lasts several months, the prices of cables, panels, and labor are never fixed. This is precisely why the BT47 index exists. Published by INSEE, this index measures the evolution of costs related to electrical work in buildings, and it can be found in most public contracts and many private agreements.
Series consistency and reference date: the trap that contracts do not signal
Before even understanding the revision formula, one must check a point that many beginners overlook: the index series used must exactly match the one specified in the contract. The BT47 belongs to the official INSEE nomenclature “Building Index – BT47 – Electricity – Base 2010”. If your contract mentions a different base or periodicity, the calculation becomes questionable.
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Specifically, three elements must be checked before any operation: the index base (base 2010 for the current series), the reference date stated in the contractual clause, and the chosen periodicity (monthly in the vast majority of cases). A discrepancy in any of these parameters is enough to distort the revised amount.
To understand the BT47 index for beginners, this consistency check is the first step, well before pulling out a calculator.
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Price revision with the BT47: applying the formula without mistakes
The revision clause follows a simple principle: we compare the value of the BT47 index at the time of the work execution with its value at the reference date of the market. The ratio between these two values gives a coefficient that we apply to the initial price.
Structure of the revision formula
The standard formula includes a fixed part and a revisable part. The fixed part (often around one-tenth of the price) does not change. The revisable part evolves proportionally to the BT47. Thus, we obtain a revised price that reflects the actual increase or decrease in electricity costs over the period.
One point to remember: INSEE publishes the values with a delay of two to three months. The value for January, for example, is only available in March or April. On an ongoing site, this means sometimes working with provisional values, then regularizing.
What the formula does not replace
The BT47 measures the evolution of a basket of aggregated costs (salaries, electrical materials, overheads). It does not replace a detailed cost estimate item by item. If the price of copper skyrockets but the other components remain stable, the index will smooth out the increase. It cannot be used to justify a one-time increase on a specific material.
Feedback varies on this point: some professionals consider the BT47 sufficient to cover current variations, while others prefer to add specific material clauses on high-cabling intensity sites.
Difference between indexing and price revision in a contract
These two mechanisms are often confused, and the confusion can be costly during an amendment or a claim.
- Adjustment occurs before the start of the work, when there is a delay between the submission of the offer and the start of execution. It adjusts the initial price to account for cost changes during this waiting period.
- Revision applies during the execution of the site. It is the one that uses the formula with the BT47 to recalculate the price at each work situation or at each deadline specified in the contract.
- A fixed price clause means that no adjustment is possible, neither before nor during. In public contracts, regulations strictly govern the cases where a fixed price is allowed.
On a project lasting a few weeks, the distinction changes little. On a long-duration project (several months, tight supply), the absence of a revision clause directly exposes the company’s margin.
BT47 and long projects: securing margins in an inflationary period
Contexts of prolonged price increases, such as those observed after recent supply crises, have highlighted the practical utility of variation clauses. Without contractual revision, an electrical company that has signed a fixed price absorbs the entirety of the increase in materials and wages.
When the clause makes a difference
On a multi-month contract with staggered deliveries, the BT47 allows billing each tranche at the actual cost of the moment. It is a financial management tool, not an administrative formality.
For private contracts, the clause is not mandatory. It is recommended to systematically include it in estimates as soon as the expected duration of the project exceeds a few months. The wording should specify the chosen index (BT47), the reference date, and the applicable formula.
Check INSEE publications
Official values are published in the Official Journal and on the INSEE website. Before each revision calculation, we check that the value used corresponds to the latest definitive publication, not an estimate. An error in the period between the initial index and the final index distorts the entire calculation.
The BT47 index is not complex once we have integrated its three fundamentals: check the consistency of the contractual series, apply the formula with the correct dates, and keep in mind that it covers a global evolution, not an isolated material cost increase. In a well-drafted contract, it is the safety net that protects both the project owner and the company.