This applet simulates a system that is owned by a power utility company. The company provides power to three communities. It owns and operates generators (which can be turned on and off in the exercise) and the transmission lines that serve the communties. On this system, it is usually possible for the utility company to supply all of the load demand using the local generators. However, there could be many reasons why this is not possible or most profitable, and then electricity is purchased from or sold to the external system.