gigawatt-hour

The gigawatt-hour is where single installations meet grid statistics. Storage is its main home: the Bath County pumped storage plant in Virginia, the largest in the United States, holds about 24 GWh behind its upper reservoir, and the biggest grid battery sites have passed 3 GWh. On the consumption side, one GWh is a year of electricity for roughly 90 to 100 average American homes.

Battery factories put the unit to a stranger use. A gigafactory rated at 35 GWh per year generates nothing; it manufactures cells whose combined storage capacity totals 35 GWh annually, so the figure is a production rate of capacity, not a flow of energy. Reading factory GWh as power-station GWh is a category error the spelling does nothing to prevent, and it appears in print regularly.

The pairing to keep straight is MW against MWh, the same distinction one prefix down: a storage project rated 300 MW and 1,200 MWh can discharge at 300 MW for four hours, and neither number alone describes it. Quoting only the MW makes a battery look like a power station; quoting only the MWh hides how fast it can deliver. The renewable energy solvers on this site run the corresponding arithmetic in either direction, capacity times hours times capacity factor, which is exactly how a 100 MW wind farm becomes the 300 GWh a year the definition on this page mentions.