The electrical grid requires continuous balance between generation and consumption. Unlike natural gas or water, electricity cannot be easily stored in large quantities. If power supply exceeds demand, this “excess generation” immediately creates an operational problem by pushing the system out of equilibrium. Maintaining this equilibrium is fundamental to grid stability, as any imbalance directly affects the grid’s operational frequency, which must be managed within tight tolerances.
Maintaining Real-Time Grid Stability
Excess electricity immediately increases the grid’s operational frequency. In North America, the system is designed to operate at a steady 60 Hertz (Hz); a surplus of power causes the generators to spin marginally faster, increasing this frequency. If the frequency deviates too far above the standard, sensitive electrical equipment and power plants can be damaged or automatically disconnect, risking cascading failures and a widespread blackout.
To counteract this, grid operators rely on frequency regulation systems and generators that rapidly adjust output. Within seconds of detecting an over-frequency event, control systems issue commands to “ramp down” or “de-load” power plants. Dispatchable generators, such as natural gas turbines, are suited for this purpose because their output can be quickly throttled back by reducing the fuel supply.
These minute-by-minute adjustments are managed by specialized entities, such as Independent System Operators (ISOs), which continuously monitor the flow of electricity. This process is a constant, high-speed negotiation to keep the system frequency within a fraction of a Hertz of the 60 Hz target. Without this immediate reduction in supply, the over-frequency would quickly destabilize the electrical network.
Converting Surplus Energy to Storage
A more productive solution than reducing generation is capturing the excess energy for later use through storage technologies. The oldest and largest form of grid-scale energy storage is Pumped Hydro Storage (PHS). PHS facilities use surplus electricity to pump water from a lower reservoir to an upper one, storing the energy as gravitational potential energy.
When electricity demand rises, the water is released back down through turbines to generate power, often achieving a round-trip efficiency between 70% and 80%. While PHS is effective for long-duration, large-volume storage, its deployment is limited by the need for specific geography—namely, two reservoirs at different elevations.
Utility-scale Battery Energy Storage Systems (BESS), predominantly using lithium-ion technology, offer a flexible and fast-response alternative. These large battery banks absorb excess power in milliseconds, making them ideal for short-term functions like frequency regulation and smoothing the variable output of solar and wind farms. Other technologies, such as thermal storage (molten salt) or compressed air energy storage, also convert surplus electricity into a form that can be dispatched later.
Redirecting Power Through Transmission and Consumption
When storage is unavailable, the grid utilizes excess electricity by moving it geographically or stimulating new demand. Electrical grids are interconnected across regions, allowing a surplus in one area to be transmitted to a neighboring region experiencing high demand or a generation shortage. This process is managed through inter-regional transmission lines, acting as high-capacity pipelines that allow the excess power to be sold or exported.
Grid operators also incentivize large energy users to increase their power consumption during times of surplus. This is driven by real-time market prices that can drop significantly, sometimes becoming negative, when supply heavily outweighs demand. Large industrial facilities, like refineries, foundries, or data centers, can earn money by running equipment or increasing operations during these low-cost periods, effectively soaking up the excess generation.
Demand response programs originally focused on reducing consumption during peak times, but the opposite action is increasingly used to absorb surplus. These programs can automate the pre-cooling of large commercial buildings or the charging of electric vehicle fleets when prices signal an oversupply, turning flexible consumption into a resource for grid balance.
When Excess Power Must Be Wasted
Despite efforts to store or redirect power, excess electricity must sometimes be wasted, a process formally known as “curtailment.” Curtailment involves the deliberate act of shutting down or limiting the output of generators, most often wind and solar farms, because the grid cannot safely handle the electricity being produced.
This necessity often arises due to physical constraints, particularly transmission bottlenecks where local lines lack the capacity to carry the high volume of power to distant consumers or storage facilities. If a buyer exists miles away, the electricity is trapped, forcing the generator to be powered down.
Curtailment can also be an economic decision, occurring when wholesale power prices drop into negative territory, meaning generators must pay the grid operator to take their electricity. A producer may choose to stop generating power rather than incur a financial loss. Curtailment represents a lost opportunity for clean energy, highlighting the need for greater transmission infrastructure and more flexible storage solutions.