Load shedding is a form of demand response whereby loads are reduced (automatically or under the control of a power system operator) in response to falling frequency. Operators must always maintain a balance of supply and demand to keep frequency at required levels. Normally, this is done by ramping flexible sources of supply — either power plants or storage — in response to changes in load. But under certain circumstances, balance is achieved by reducing loads rather than increasing supply.
Forms of load shedding include both voluntary and involuntary types:
| Type | Description | Voluntary/ Involuntary |
| Frequency responsive demand response | Some loads can be interrupted with no notice. These loads can be connected to a relay that measures system frequency locally and reduces load in direct response to falling frequency. In this case, the load will shed immediately and automatically when frequency drops below a specified set point. | Voluntary since loads chose to participate in DR program |
| Automated demand response | Some loads can be reduced in response to a control signal from a system operator, utility, or aggregator. Loads typically participate in demand response programs in return for financial incentives such as rate discounts or direct payments. | Voluntary since loads chose to participate in DR program |
| Under-frequency or over-frequency load shedding relays on the transmission/ distribution grid | To avoid damage to the grid under rapidly changing frequency conditions, under-frequency and over-frequency protective relays are placed at interconnections between the transmission grid and distribution substations. When triggered, these relays automatically disconnect distribution circuits resulting in shedding of all loads on the affected circuits. | Involuntary |
| Rolling blackouts | A rolling blackout is a planned process where utilities disconnect specific circuits to reduce overall system load for a limited amount of time, then disconnect different circuits and reconnect the original circuits. This allows outages to be spread across the system without requiring any one customer to be out of power for too long. Often rolling blackouts are announced to the public ahead of time, although they may occur on short notice. Rolling blackouts are used when there is insufficient system supply to meet demand and no other sources of supply are available. | Involuntary |