PG&E plans to use mostly cheap off-peak night-time wind energy to pump compressed air into an underground reservoir. The pressurised air will be released during hours of peak demand to augment natural gas power plants operated by PG&E. The company says the output capacity could provide the equivalent of 300 MW for a ten-hour duration. Energy storage remains a disputed topic in the wind industry, with many experts arguing that enough flexibility and control already exists in most power grids to easily cope with the variable nature of wind energy production. "There are a lot of really strong opinions on that," says Ryan Wiser, a staff scientist at the Lawrence Berkeley National Laboratory. "To the extent that California has very aggressive renewable energy targets that involve a lot of renewables generation, certainly storage of that nature begins to have a greater value. The real question is whether the system value of storage is sufficient to absorb the added cost."