This study supposes an enterprising firm that signs a contract to serve as an energy management (EM) provider with an entire residential community．Under the contract, the energy service provider (ESP) meets the daily demands for electricity that hourly varies erratically and dynamically．The ESP's strategic decisions are the investment scales for various energy facilities such as the gas engine generator, renewable energy generator and energy storage facility as well as the procurement capacity contract with the main grid supplier like the utility company．This paper formulates the problem as a stochastic programming and present some numerical examinations on system design and operations．The examination demonstrates the unit investment cost for the battery facility as a key to the EM system composition．As the investment cost for the battery gets lower, the firm is shown to introduce more of renewable energy sources and to be less dependent on the procurement from the main grid supplier.