Performance, assessment of model predictive controllers is a problem of significant industrial relevance. Model predictive controllers, belong class of linear time-varying controllers, which compute the future control actions by, a constrained, time-varying objective function. In this work we propose a performance statistic that takes into account the time-varying and. constrained nature, of model predictive control. The proposed, measure, compares the achieved objective function with its design value, online. Analytical expressions are derived to calculate the expected value of the design objective function under closed loop conditions-Simulation and industrial case studies are used to illustrate applicability of the proposed metric.