By Katie Fehrenbacher, GigaOMThe longstanding argument for why plug-in vehicles aren’t that green is that if the electricity grid is powered mostly by coal, well, then so are our plug-in cars. That’s not so great when it comes to reducing carbon emissions. But the ideal is that over time as consumers and corporations increasingly embrace EVs, the power grid will also correspondingly shift over to incorporating clean power, like solar and wind. And in the meantime, some utilities can offer green power services for EV drivers. Well, those are the visions. However, there are major hurdles to implementing these ideas. Here are the road blocks: Infrastructure Investment First and foremost, there will be a colossal investment needed for both clean power and electric car infrastructure to make their way onto the market, and both will take a lot of time. Author and professor Vaclav Smil has explained in his recent book that an all-electric U.S. fleet would conservatively need 980 TWh of electricity per year to run, which was 25 percent of the U.S. electricity generation in 2008. Smil thinks utilities wouldn’t realistically be able to build that additional amount of electricity generation within two decades. In addition, that extra generation would have to come from clean power to be carbon-reductive. As anyone who has followed the utility-scale solar market knows, it takes years for utility-scale solar projects to move from drawing board to supplying electricity. In the case of BrightSource Energy’s inaugural solar thermal project Ivanpah, it has taken over three years to just get regulatory approval, and now here come all the environmental protests. California’s utilities have struggled to meet the state mandate that says they need to buy 20 percent of their electricity supply from clean power by 2010. Most utilities weren’t likely to make that deadline, but state regulation gives them until the end of 2013 to comply. Meanwhile, the utilities will have to make sure they line up enough contracts or install their own projects to meet the 33 percent goal by 2020. And this is just in California, which has an aggressive state mandate.