Appropriate determination of reactive power requirements is an important issue for maintaining voltage stability， decreasing transmission losses， and mitigating the potential exercise of reactive market power. A model is developed for annual procurement of reactive power capacity considering the possible outage of multiple VAR sources in an electricity market environment. The objective is to minimize the total cost， including the cost of purchasing reactive power capacity， load shedding and generation redispatch. A method based on the Monte-Carlo simulation and differential evolution algorithm is used for solving the established optimization problem. Simulation results from case studies demonstrate the essential features of the developed model and the solution method.