As Tesla's share price sank below US$200 ($305.20) this week, Elon Musk lost one of his most effective levers in keeping the electric-car company motoring.

The group's stock price has been a powerful competitive weapon for Musk, enabling him to raise large amounts of money relatively cheaply.

But a series of setbacks, from disappointing earnings figures to questions over demand for its electric cars, and a series of fires in its vehicles, have weighed on investors' sentiment.

Shares are down 40 per cent this year alone, and currently trade close to their lowest level in two and a half years.