Most people worry about their computer's energy use when it is on. But a new study shows that most of the energy a computer ever uses is in the manufacturing process.

Researchers at Arizona State University and the Rochester Institute of Technology did a life cycle assessment of a typical laptop computer (in this case a Dell Inspiron 2500). They found that the laptop requires 3,010 to 4,340 million joules of energy to make and emits about 227-270 kilograms of carbon dioxide in the process. Carbon dioxide, a greenhouse gas, is one of the primary contributors to climate change.

That's about as much as the average car does after driving 550 miles, or flying in a jet plane from New York to Philadelphia. It represents about 62 to 70 percent of the total energy the computer consumes during its entire lifespan.

There is a wide range in the amount of carbon dioxide produced because computers use different manufacturing processes for their components, and the supply chains are complicated, the researchers say. But the results show that it may the best place to attack energy efficiency is at the factory rather than the desktop.

Callie Babbitt, a co-author of the study and an assistant professor at the Golisano Institute for Sustainability at RIT, told that the energy used in building laptops for the U.S. market alone was the same as driving 676,000 cars for a year.

Even accounting for complicated supply chains, the materials used to make a laptop only accounted for about 10 percent of the carbon emissions in the manufacturing process.

Besides using more energy-efficient manufacturing, the authors say, a good way to deal with energy use is to make computers more easily upgradeable. That reduces the need for new computers and could make a sizeable dent in the total energy usage by the industry. That would increase the environmental benefits that come with using computers to manage efficiency in other areas, such as traffic control.