A plug-in electric car develped by a start-up company in the United States is leaving its conventional rivals behind. The Tesla Model S has been named Car of the Year by Motor Trends magazine, in one of the auto industry's most prestigious, closely watched awards. It is the first time a battery-powered vehicle has scooped the coveted accolade. But questions remain as to whether electric cars can provide a realistic and commercial alternative to conventional vehicles, and whether they really are that green.