A blackout is a complete interruption of power in a given service area. Rolling blackouts are controlled and usually preplanned interruptions of service. A brownout is a partial, temporary reduction in system voltage or total system capacity.
Blackouts come without warning, last for indeterminate periods, and are typically caused by catastrophic equipment failure or severe weather. The nature and cause of the blackout determines who is affected.
Rolling blackouts typically occur with at least some advance warning, normally last for a fixed length of time, and are deliberately produced by utility companies. They can be used as a means of coping with peak power demands that cannot be met from existing supply. Rolling blackouts are usually intended to affect only a specific service area, and the energy provider will typically spread these blackouts among several service areas to insure that no specific area suffers substantially more than any other. Planned outages and rolling blackouts differ slightly in that planned outages are usually announced well in advance and are most commonly needed to allow for routine maintenance, while rolling blackouts can occur with relatively little warning and are intended to take stress off of the system's energy load.
In most cases, brownouts are deliberately produced by energy providers as an emergency measure to prevent the system from failing completely (blacking out). Typically a utility will decrease system voltage by 10-25%, usually for a short period of time. This reduction typically has minimal effect on heat and lighting systems, most of which can function reliably for short periods on suboptimal voltage, but sensitive electronic equipment requiring reasonably precise voltages may not be able to function and long-term brownouts can cause premature wear in non-electronic devices. Computer disk drives often suffer write failures when supplied with suboptimal voltage, and electric motors tend to run hotter when required to produce the same horsepower during a brownout.
Normal fluctuations in voltage do not qualify as brownouts. System voltage in many service areas can vary by as much as five percent above or below "nominal" line voltage. Manufacturers of electrical and electronic products know this. Most North American consumer and commercial products are designed to function normally and safely for long periods at voltages ranging from 115 to 125 volts.
See also:power, volt, peak demand, peak supply, peaking capacity, sag