Elon Musk, the chief executive of Tesla, announced on Twitter on Sunday that his company would build a factory in Shanghai with the aim to assemble 10,000 giant batteries annually for electric producers and distributors. The batteries, which Tesla calls Megapacks, are designed to store large amounts of electricity — a single Megapack can power 3,600 homes for one hour, according to Tesla. The batteries, which are roughly the length and height of an international shipping container, can discharge the electricity to run factories or homes when demand from the local power grid is high, or during a blackout. The capacity to store electricity when it isn’t in demand is critical as electric utilities move toward wind and solar energy to replace power generated by fossil fuels. In China, demand for grid-storage batteries is especially strong. Many provinces now require new solar and wind farms to have enough batteries to hold 10 to 20 percent of the electricity they generate.