The balancing of electric supply and demand

When the demand for electricity nears available supply levels, it is sometimes necessary to temporarily interrupt the delivery of electricity to maintain the integrity of the electric grid and to prevent catastrophic grid failures and extended outages for customers. This is called “shedding load.” 

Shedding load may occur if there is a shortage of electricity supply, or to help prevent power lines from becoming overloaded. Several factors can lead to load shedding, including extreme weather, sharply increased electric demand, unplanned generation plant outages, transmission constraints, unexpected damage to equipment, unavailability of purchased power or a combination of these situations.

Shedding load is always a last resort, but when necessary this action helps prevent more extensive and prolonged power outages that could severely affect the reliability of the power grid for weeks or even months.

Why is shedding load necessary?

Shedding load is required when the demand for electricity approaches supply, creating the potential for a dangerous imbalance. It’s a way to help reduce power demand by turning power off to some customers to help prevent longer, larger outages. The immediate reduction of power demand is critical to prevent a catastrophic, extended failure of the larger electrical grid. 

Power generation and load must always match up, or remain balanced, otherwise the grid’s integrity will be compromised. There are strict standards that utilities must follow to maintain this balance.  Shedding load is always a last resort to prevent more extensive and prolonged power outages that could severely affect the reliability of the power grid.

For example, in 2003, the biggest blackout in North American history plunged 50 million people into darkness for 31 hours.  In that blackout, a high-voltage power line in northern Ohio brushed against some overgrown trees and tripped off-line. Over the next hour and a half, three other high voltage lines sagged into trees and tripped off-line, forcing other power lines to shoulder an extra burden. The overtaxed lines cut out or tripped, causing a cascade of transmission line throughout southeastern Canada and eight northeastern states.

Does this mean all customers will be affected if a load shed is required?

Not necessarily. More than 16,000 circuit miles of transmission lines run through Entergy’s four-state service area, and the company operates more than 40 power plants. Shedding load is always a last resort, but if needed, could affect different customers depending on the cause or situation:

• Localized: This could occur if there’s unexpected damage to infrastructure in a specific geographic region. For example, damage to a major transmission line, such as a tree falling and taking it down, or if there’s an unexpected outage at a generating plant, producing a shortfall of energy to the area. This scenario usually impacts a smaller, more localized set of customers, but could also span a larger geographic footprint (within an Entergy operating company) depending on the conditions.
• Across the Service Area: Entergy’s grid is managed by our reliability coordinator Midcontinent Independent System Operator. A systemwide load shed is initiated only by MISO as a result of an imbalance of supply and demand of electricity and required to protect the bulk electric system (which is interconnected across the country). Such a scenario could occur due to extreme weather, such as a hurricane, winter storm, or high or low temperatures. Entergy has only been required to shed load across our service area once since we joined MISO in late 2013, and this load shed occurred during the extreme cold weather brought by Winter Storm Uri in February 2021. Uri impacted much of the country, resulting in record-breaking low temperatures.

How does load shed work?

Depending on the type of constraint and given impacts to the grid – localized or systemwide – either Entergy or MISO will initiate steps to shed load. For example, if MISO initiates, Entergy is ordered to begin shedding load under emergency conditions. This is a requirement to avoid catastrophic grid failures which could result in prolonged power outages. In fact, if we don’t shed enough load in time, MISO will shed the required amount of load on our behalf.

Since shedding load is always a last resort, MISO, for example, has other measures it may take to try and overcome a power shortfall, such as importing more power from other resources or tapping into emergency reserves. It can also order its members, like Entergy, to make a public appeal to customers to voluntarily reduce their energy consumption to prevent load shed.

Why are some customers more affected than others?

Required outages during a load shed event limit power to some customers who are grouped together. Power in the group’s electricity conductors is turned on and off to their homes or businesses. And, we typically rotate the outages until the load shed is complete to minimize the burden on any one group of customers.

• The groups are determined by the amount of power, or megawatts, that must be shed at the time the outages occur and can vary greatly depending on the current conditions.
• We do our best to avoid critical customers, including essential public health and safety services and facilities, such as hospitals, fire and police departments and water systems.
• As one group of customers completes its outage cycle, the next group is removed from service and the first is returned to service. Isolated cases may occur when a manual circuit breaker operation is required to restore service at the end of an outage cycle. This takes longer than the normal automatic breaker operation.  

Why can’t you provide more notice before shedding load?

While we try to provide as much notice as possible to customers in advance of a load shed event, unfortunately we are not always afforded time to do that.  We do, however, communicate to customers when we believe there could be conditions leading to load shed.

As we’ve shared, shedding load is implemented as a last resort and requires immediate action. When the MISO directive to shed load is received, our customer service teams begin the process of alerting customers. Outages follow quickly, often before notifications can be made. This could happen at any time, day or night.

We are typically provided little notice and must comply with an order from MISO for shedding load to help prevent longer, widespread uncontrolled outages. Visit here to learn more about MISO’s emergency messaging and the associated actions their members take.

Can you tell me more about MISO?

Entergy has been a member of the Midcontinent Independent System Operator, one of the nation's largest regional transmission organizations, since December 2013 and is part of their southern region. MISO is a not-for-profit, member-based organization that ensures reliable, cost-effective delivery of electricity across all or parts of 15 U.S. states and one Canadian province. In cooperation with stakeholders, MISO manages approximately 65,000 miles of high-voltage transmission and 200,000 megawatts of power-generating resources across its footprint.

Being a part of MISO allows Entergy to better coordinate and optimize generation and transmission for the benefit of Entergy customers. Learn more about MISO operating conditions here.

What about storing power to reserve a surplus? Why don’t you turn off power to industrial customers or turn off street lights to help reduce load demand?

Technologies exist that allow energy to be stored for future supply, but to date have not been cost-effective compared to alternatives. As these technologies continue to emerge, and in the future become more cost-effective alternatives to other types of investments to enhance resilience, they may play a role in preventing shedding load.

Entergy has agreements in place with some industrial customers allowing us to request a reduction in the amount of energy they are consuming to a pre-determined amount in an emergency situation such as a load shed event. We also have certain residential and business customers that will participate in voluntary demand response programs and will reduce load to help avoid a load shed.  These voluntary actions by participating customers serve as a last line of defense before having to interrupt power to our residential and business customers.

Reduction in curtailable load or calls for conservation could avoid the need to shed load to a broader group of customers, however, should conditions continue to escalate after this step it still helps reduce the overall quantity of megawatts required to be shed. This means fewer customers are potentially impacted.

In regard to streetlights, they make up a very small percentage of our electric demand, and we do not have the ability to remotely turn them off.

What can I do to help prevent a load shed event?

When MISO orders a public appeal for conservation, we will ask customers to minimize energy usage as much as possible until the system strain has passed. We may ask for your help in conserving energy prior to a public appeal, given the conditions. You can help by:

• Increasing or decreasing your thermostat, depending on the weather.
• Unplugging electronic devices and turning off lights that are not in use.
• Holding off on doing chores. Delaying laundry, washing dishes and other non-essential uses of electricity.
• If you have all electric appliances, washing clothes with cold water, shower quickly instead of taking a bath, cooking foods at the lowest possible setting and refraining from opening the oven door while baking. 
• View here for more tips on how to reduce your energy usage.

Call 1-800-9OUTAGE (1-800-968-8243) to report downed power lines