How the power grid keeps up when demand changes

Switch on a lamp at dusk and its demand joins all the other changes happening across the power network. Houses turn on lights, a factory may finish a shift, and generators may be producing a different amount from the afternoon. Grid operators have to keep electricity supply and use in balance as those changes unfold.

The route from generator to outlet

The U.S. Energy Information Administration describes a network of power lines, substations and transformers. Transmission lines move electricity over long distances at high voltage. Transformers change the voltage along the way, and local distribution lines bring electricity to homes and businesses. How much electricity can be generated and how much the network can deliver to a particular place are separate questions.

An area with spare generation can sometimes help its neighbours. That depends on the connections between the areas and on how much power those lines can safely carry at the time. A region-wide total therefore tells only part of the story of delivery at a particular location.

Operators prepare for demand and respond as it changes

The U.S. Department of Energy describes how balancing authorities forecast demand, schedule resources and coordinate transfers. A forecast gives operators a starting point for planning. When use or supply departs from that plan, they can adjust generation and transfers. Available storage and demand reductions can also help.

In a shared alternating-current system, frequency is one signal of imbalance. If demand exceeds supply, frequency can fall below its normal level. Operators maintain reserves and use several kinds of response to keep the system within operating limits. A reserve is capability they can call on, subject to how quickly and for how long it can respond.

A battery rating comes with a time question

Imagine a charged battery rated to deliver 10 megawatts and supply 20 megawatt-hours. In an idealised calculation it can supply 10 megawatts for two hours. A second battery rated at the same 10 megawatts with 5 megawatt-hours can sustain it for half an hour. Megawatts describe the rate of delivery. Megawatt-hours describe the amount of energy.

Those examples assume that the stated energy is available at the rated output. In operation the battery's starting charge and operating limits matter. The next time a report describes a new resource, look for how much power it can deliver, how long it can sustain that output, and whether the network can get it where it is needed.