In recent years, interest in smart grids has increased globally along with the trend to reduce greenhouse gas emissions. One of the smart grid research area actively being done is energy trading in a microgrid. In particular, an energy trading system in the microgrid within an island should consider the own energy independence of the microgrid very seriously. In this paper, we propose a new metric to evaluate the energy independence of an energy trading system in a microgrid. The energy independence is discussed through energy trading simulation, which is first performed in this paper. Furthermore, we numerically present the proper energy trading cycle under various conditions with an existing energy trading mechanism.