http://techcrunch.com/2009/06/23/the-government-comes-throug...
Coal is majority of the U.S. power grid: http://www.ucsusa.org/clean_energy/coalvswind/c01.html
Tesla batteries are cleaner than older versions, but they contain some Cobalt, and Lithium has many impacts and risks for humans.
Also, Tesla is a public company and therefore discloses their finantials. Their total debt is 270M, and they have 280M in cash (therefore, negative net debt).
http://www.businessweek.com/news/2012-05-11/tesla-to-begin-r...
And to point out the obvious, the Federal government extracts far more money from the oil companies (and hence oil consumers) by way of gas taxes than it pays out in subsidies.
This payoff can't be captured by a private investor, so the government has incentive to invest earlier (and at higher risk levels) than private investors.