A microgrid is an energy subsystem composed of generation units, energy storage, and loads that requires power management in order to supply the load properly according to defined objectives. This paper proposes an online energy management system for a storage based grid-connected microgrid that feeds a critical load. The optimization problem aims to minimize the operating cost while maximizing the power provided by the renewable energy sources. The power references for the distributed energy resources (DER) are scheduled by using CPLEX solver which uses as input current measurements, stored data and adjusted weather forecast data previously scaled in each iteration considering the current status. The proposed structure is tested in a real time simulation platform (dSPACE 1006) for the microgrid model, by using Labview to data acquisition and Matlab to implement the energy management system. The results show the effectiveness of the proposed energy management system for different initial conditions of the storage system.