NEW YORK (MarketWatch) -- Standard & Poor's on Tuesday published an unsolicited rating for electric carmaker Tesla Motor Inc. and said there is enough investor interest in the company to warrant analytical coverage. Ratings agencies are usually paid by issuers of debt to assign them a rating, but S&P does not have an agreement with Elon Musk's Tesla, it said in a statement. The agency assigned the company a B- rating, placing it three notches below investment grade, with a stable outlook. "Our "vulnerable" business risk profile assessment incorporates Tesla's narrow product focus, concentrated production footprint, small scale relative to its larger automotive peers, limited visibility on the long-term demand for its products, and limited track record in handling execution risks that could arise in managing high volume parallel production," S&P said in a statement.