Stated simply, the ultimate objective of the transmission system is to deliver electric power reliably and economically from generators to loads. Power systems are large, highly complex, ever-changing structures that must respond continuously in real time. Electricity must be produced and delivered instantaneously when it is demanded by a load. Power outages are not acceptable, so the system must also tolerate sudden disruptions caused by equipment failure or weather. And the system must perform as economically as possible, with transactions and sales monitored accurately.