Electricity is unique in that it must be produced the instant it is needed. It just cannot be economically stored in large quantities using today's technology. And, unlike telephone service, power users do not tolerate a busy signal.
The utility normally tries to schedule and operate a series of generators at a given power level known to be the most efficient for the season and time. As the customers' needs for power change, the dispatchers must adjust the amount of electricity produced by various generation units. If the required load is higher than what the generators are currently producing, the system's electrical frequency falls below the desired value of 60 cycles. The generation dispatchers decide which unit is the most economical to increase power output, or they bring an additional generating unit on line to raise the frequency back up to 60 cycles. If the generators are producing too much electricity compared to what the required load is, the system frequency increases above 60 cycles and the generation dispatchers decide which unit's power generation needs to be lowered.
In the event the electric utility doesn't generate the required power when it is needed, the system voltage drops below the minimum setpoint, and circuit breakers begin to trip to prevent equipment damage. This can cause major power outages for consumers. Fortunately, today's electric utilities have interchange agreements with other power suppliers that minimize the likelihood of this happening.
Electric utilities also plan their power generation to meet widely varying demands during the year, and during any given day. Because of the varied schedules on which customers use electricity, the load varies over the day, the week, and the year. Normally, the load tends to be lowest at night, when most people are asleep, and highest during the day, when the most appliances are in use. Some utilities see a peak at night due to electric space heating in the colder months, while most utilities see a peak during the hottest days of the summer when air conditioners are working their hardest.
When a utility has its highest demand for power in the winter, it is referred to as winter peaking. When the demand for power is highest in the summer, it is called summer peaking. Some utilities have such dramatic load growth that they have both.