Tesla's (TSLA 3.17%) business of building really big batteries just keeps getting bigger.

Today, the company most famous for building electric cars, but that's also getting increasingly famous for building big batteries (for utility-scale storage of electricity), announced its latest project. In cooperation with California electric utility Pacific Gas and Electric Company (PCG 0.43%), Tesla will build a 182.5 megawatt (MW) lithium-ion battery energy storage system at the PG&E substation in Moss Landing, Monterey County, Calif.  

Tesla Big Battery electricity warehouse in a field in foreground and windmills in the background

Tesla's battery warehouse project in Australia, smaller than the one being built for PG&E. Image source: Tesla.

182.5 megawatts refers to the maximum amount of energy output that the battery energy storage system's (BESS) 256 Tesla "Megapack" battery units can release onto the grid at times when extra power is needed (for example, to forestall a brownout or blackout when power consumption is high). The total actual storage capacity of the "BESS" is stated as 730 megawatt hours (MWh).

Thus, once complete, the BESS will gradually store up the 730 MWh of power and, when needed, be able to release 182.5 MW per hour, sustaining the grid with its saved-up juice for four full hours.

The companies say the Moss Landing BESS will be "one of the largest utility-owned, lithium-ion battery energy storage systems in the world." To put it in context, it will be about 22% bigger than the already-expanded battery warehouse that Tesla built in Australia two years ago -- a big leap forward for renewable energy.

Tesla and PG&E will cooperate on the design, construction, and maintenance of the BESS, but it will be owned and operated by PG&E once finished. The companies broke ground on the project on July 21, and expect to have it up and charged to capacity by early 2021, and "fully operational" by the second quarter of 2021.