One advantage of being an automaker that produces nothing but plug-in vehicles is earning zero-emissions credits. It's long been thought that those credits would at some point become a valuable commodity. Back in 2007 when Phoenix Motorcars was still around, a big chunk of its business plan revolved around revenue from those credits. Phoenix went bust before they were ever able to deliver any vehicles, but Tesla Motors has picked up the idea.

Buried in an updated S-1 filing with the Securities and Exchange Commission this week, Tesla revealed that it has sold $13.8 million worth of zero emissions vehicle credits, representing 368 cars, to Honda over the last two years. Honda and Tesla also have an agreement to exchange credits for another 287 vehicles. Unfortunately for Tesla, the market for its credits will soon largely dry up as Roadster sales have slowed to a trickle and the Model S won't come to market for at least two more years. For Honda, there will be plenty of other sources for these credits – including Nissan, General Motors, Ford and others – beginning later this year.

[Source: Securities and Exchange Commission via Green Car Advisor]