By Sophie Knight TOKYO (Reuters) – Panasonic Corp confirmed on Thursday it would invest in U.S. electric carmaker Tesla Motors Incs planned $5 billion lithium-ion battery plant in the United States, but said it had yet to decide on the size of its investment. The Japanese company, which already supplies batteries for Tesla, said it was gauging the demand for batteries before deciding on an amount. We have not yet decided exactly how much we will invest and when, said Chief Financial Officer Hideaki Kawai. However, Tesla is a very important partner to us and discussions are continuing.