Today, the US power grid is the world's largest machine. It contains more than 7,300 electricity-generating plants, linked by some 11 million kilometers of powerlines, transformers and substations. Power grids span Earth's continents, transmitting electricity around the clock. They're massive feats of engineering but their functioning depends on a delicate balance. Their components must always work in unison, maintain a constant frequency throughout the grid, and match energy supply with demand. If there's too much electricity in the system, you get unsafe power spikes that can overheat and damage equipment. Too little electricity and you get blackouts. So, to strike this balance, power grid operators monitor the grid from sophisticated control centers. They forecast energy demand and adjust which power plants are active, signaling them to turn their output up or down to precisely meet current demand. By considering factors like the availability and cost of energy resources, grid operators create a "dispatch curve," which maps out the order in which energy sources will be used. The grid defaults to using energy from the start of the curve first. Usually, the resources are ordered by price. Those at the start tend to be renewables because they have much lower production costs.