Peak demand, also called peak load, refers to the maximum amount of electric demand created by a customer or a group of customers during a specified time period. For instance, daily peak demand refers to the maximum amount of power used during a specified day. For many utilities, peak demand occurs in the late afternoon or early evening. System operators must ramp supply sources as loads grow and must plan resource availability so that enough supply is available to meet the system peak. The sources of supply turned on or ramped up during peak hours are often called peaking generation.