The drive to grow biofuels for transport has focused on converting crops to ethanol which can be used in internal combustion engines. However, that is the wrong approach, according to a study published today in the journal Science: it is much more efficient to convert biomass to electricity for battery-powered vehicles.

The authors, from the Carnegie Institution at Stanford and the University of California, Merced, calculated that generating electricity, by burning biomass in an efficient power station, delivered 80 per cent more mileage per acre of crops than conversion to ethanol for liquid fuel. It also doubled the greenhouse gas offsets to mitigate climate change.

"It's a relatively obvious question once you ask it, but nobody had really asked it before," says Chris Field of the Carnegie Institution.

Bioelectricity was a clear winner over bioethanol, whether the energy came from corn or switchgrass (a new cellulose-based crop).

A car powered by bioelectricity could travel almost 14,000 miles on the net energy from an acre of switchgrass, while a car powered by bioethanol from the same crop would go only 9,000 miles.

"The internal combustion engine just isn't very efficient, especially when compared with electric vehicles," says Elliott Campbell, another author. "Even the best ethanol-producing technologies with hybrid vehicles aren't enough to overcome this."