The design and implementation of the French energy efficiency certificate provides an opportunity to test the hypothesis of the performativity of the economic theory associated with the cap-and-trade market-based instrument. The empirical study of its design and implementation, through interviews and regulatory analysis, reveals a progressive dynamic of decoupling from its initial principles, in particular the equivalence between the quantity of energy saved and the allocation of certificates. Recent adaptations of the instrument attempt to integrate an alternative definition of efficiency based on cost estimation and subsidy allocation. By focusing on calculation practices, this chapter contributes to the understanding of performative struggles and theory-practice decoupling associated with market-based instruments. The chapter identifies four conditions for theory-practice decoupling: competing theories of efficiency, attachment to the instrument, a new principle of action, and weak checks on conformity to theory.