Energy Systems / EXPLAINER
The Two Numbers Every Battery Story Needs
Power tells you how quickly a storage system can work. Energy tells you how long it can keep going. A useful plan needs both, plus a way to recharge.
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A battery has more than one size
A description of a large battery can sound precise while leaving out its most useful characteristic. A megawatt measures a rate of delivering power. A megawatt-hour measures an amount of energy. A storage system needs both ratings because a large reservoir does not necessarily have a large outlet, and a large outlet does not tell you how much remains in the reservoir.
The U.S. Department of Energy's solar-storage guidance explains that different combinations of power and energy serve different tasks. Brief fluctuations call for a different design from an extended gap in generation. Before comparing technologies, define the job: smoothing a variable source, shifting energy into the evening or keeping selected equipment running through an interruption.
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