By Joseph White

SPARKS, Nevada (Reuters) - Tesla Motors Inc's <TSLA.O> "master plan" to broaden the electric vehicle maker's product line could cost "tens of billions" of dollars over time and would likely require a "modest capital raise", Chief Executive Elon Musk said on Tuesday.

Speaking at a formal unveiling of Tesla's $5 billion battery "gigafactory" outside Reno, Nevada, Musk said the new plant being jointly developed with Japan's Panasonic Corp <6752.T> could ultimately support 1.5 million electric vehicles a year.

Musk said he was confident the partners could eventually lower battery costs to $100 per kilowatt-hour by 2020, well below estimates by competitors.

The immediate plans call for Tesla's new Model 3 sedan, due out next year, to reach an annual production level of 500,000 by the end of 2018, Musk said.

Eventually, he expects the Model 3, which will be priced from around $35,000, to generate $20 billion in revenue per year and $5 billion in gross profit.

Last week Musk unveiled an ambitious plan to expand the company into electric trucks and buses, as well as car sharing.

(With additional reporting by Paul Lienert in Detroit and Rishika Sadam in Bengaluru; Editing by Jeffrey Hodgson)