Earlier this month, the Pacific Gas and Electric Co. launched its Tesla energy storage system at substation in Browns Valley (Yuba County), in California.

The one used by PG&E is smaller unit relatively speaking - 2 MWh of energy and 0.5 MW of power (for up to four hours), consisting of 22 Powerpacks.

PG&E i-MiEV

PG&E i-MiEV

The goal of this particular installation is to charge batteries at off peak hours and then return that electricity during peak demand times.

Another joint project between Tesla and PG&E utilizes decentralized Powerwalls for energy storage:

"PG&E and Tesla, for example, are also collaborating on a pilot project that uses Powerwall batteries in homes and businesses in the Bay Area. The amount of energy those scattered batteries can store will be equal to the amount stored at the Browns Valley substation, Della Penna said. PG&E will be able to study how both setups — one centralized, the other diffuse — affect the grid.

“They’re each with their own challenges and opportunities,” he said. “We’ve structured it so we’ll have a lot of really good learning here.”"

source: SFGATE