According to a new report from the University of Illinois at Chicago the answer is "less now than ever." According to the report, the energy needed to make a gallon of the corn-based fuel dropped by about 30 per cent in the past 10 years.

The results come from a survey of 150 dry mill ethanol plants that produce 85pc of the biofuel in the United States.

The work was conducted by Steffen Mueller, principal research economist at UIC's Energy Resources Center.

The findings have value for policy makers studying the pros and cons of fuels based on their "full life cycle" - or the total energy needed to create the fuel compared to its energy output.

In a press statement, Mueller noted that policy makers should pay attention to lifecycle analysis of greenhouse gas emissions of biofuels, but added that biofuel refineries, including corn ethanol plants were in a rapid "innovation phase".

He said the survey showed the adoption of new technology reduced energy production needs.

"The challenge for policy makers will be to keep up with these developments so that regulations are meaningful and reflect state-of-the-art industry practices," he added.

Mueller found that plants use 28pc less thermal energy and 32pc less electricity to turn corn into ethanol.

The results were gathered as part of a 24-question survey developed with ethanol industry input.