Traditionally, power system flexibility in economic dispatch is measured by the ramping up/down capacity with given total demand and locational distributional factors in a system. In this paper, we present the elasticity metrics for the optimal security-constrained economic dispatch (SCED) restoration using the sequential convergence speed of a deformed SCED. The degree of the residual deformation of the SCED solution is denoted by the norm of the Euclid distance between the optimal dispatch point without initial point and some trajectory constraints and sequential dispatches starting from the current operating point. The mathematical proof of the convergence of the sequential SCED is given in this paper. Numerical examples are based on a SCED simulation tool running on a practical power system to verify the convergence and calculate the metrics of the proposed elasticity.