storage duration

A battery is described by two different numbers, and confusing them is the commonest mistake in reading a storage headline. The first is power, measured in megawatts or gigawatts, which is how fast the battery can push electricity into the grid at any one moment. The second is energy, measured in megawatt hours or gigawatt hours, which is how much it holds in total. Storage duration is the second divided by the first. The US Energy Information Administration puts it plainly: a battery's duration is the amount of time it can contribute electricity at its nameplate power capacity until it runs out, calculated as the ratio of energy capacity in megawatt hours to power capacity in megawatts.

So a battery rated at 1 megawatt that holds 2 megawatt hours has a duration of two hours. That number decides what the battery is useful for. A two hour battery can cover an evening peak, when the sun has set but demand has not. It cannot cover a week of still, cloudy weather. American utility scale batteries built through 2020 averaged about three hours. Projects that run for six hours or more are usually called long duration storage, and they remain a small share of what gets built.