By Miho Uranaka

OSAKA, Japan The battery arm of Japan's Panasonic will need to build around four more factories to meet a plan to sharply boost annual capacity of batteries for electric vehicles by 2031, its head of technology told Reuters.

The comments from Panasonic Energy Chief Technology Officer Shoichiro Watanabe mark the first time the Telsa supplier has given a clear indication of the number of additional factories it will need.

It could also fuel expectations of more investment by Japanese companies into the United States, after Washington and Tokyo struck a deal in March that is key to giving Japanese manufacturers wider access to U.S. electric-vehicle (EV) tax credits.

Panasonic Energy said in May it aims to boost annual EV capacity to 200 gigawatt hours (GWh) by early 2031, around four times greater than its capacity as of March this year. It has a plant in Nevada and is building a second one in Kansas that once operational is expected to bring annual capacity to 80 GWh, it has said.

"We will need to build around another four factories," Watanabe said in an interview at Panasonic Energy's headquarters in Osaka on Friday.

He stopped short of mentioning specific locations, time frames or investment sizes. He also signalled an openness to potential joint ventures for EV battery production - including with automaker Mazda Motor - citing the changing nature of such projects, where the investment is no longer shouldered by battery makers alone.

"The style where battery producers will make all investments is disappearing," he said.

The 200 GWh capacity is the minimum necessary to be a major player in the space, Watanabe said.

(Reporting by Miho Uranka; Writing by Daniel Leussink; Editing by David Dolan)