We present a bi-objective two stage stochastic programming model for optimal sizing of a grid-connected hybrid renewable energy system. In this system, solar and wind are the main electricity generation resources. National grid is assumed to be a carbon-intense alternative to renewables and used as a backup source to ensure reliability. Storage device is included to examine its role in reducing the carbon emission and the intermittency of renewable sources. It is assumed that decision maker is sensitive to both cost and carbon emission, therefore two objectives are considered: total cost and carbon emission caused by electricity purchased from the utility grid. A simulation optimization algorithm has been developed for the problem.