Most people assume that remanufactured products—such as retread tires, refilled ink-jet cartridges, or rebuilt engines—are a good way to save energy. But as is so often the case in life, it’s not quite that simple.

When a team led by mechanical-­engineering professor Timothy Gutowski, PhD ‘81, examined 25 case studies on products in eight categories, they found that remanufacturing required more energy nearly as often as it saved energy. For the plurality of the items, the balance was too close to call.

The reason turns out to be simple, though perhaps not obvious: how a device gets used tends to outweigh how it was made.

The MIT team looked at the total energy used over the lifetime of a product, rather than just the energy used in the production of components and the manufacturing process. As expected, in virtually all cases it takes less money and energy to remake a product from the recycled “core”—the reusable part of the product—than to start from scratch. But the catch is that many of these remade products are less energy efficient, or could be replaced by newer versions that are more efficient, so the extra energy used over their lifetime can greatly exceed the savings from their manufacture.

Often, Gutowski says, the availability of new technology makes it more energy efficient to replace an old device than to repair or remanufacture it. For example, many new appliances—such as refrigerators and washing machines—are so much more efficient than older models that in terms of energy use, newer is almost always better.

But Gutowski emphasizes that energy consumption is not the whole picture. Remanufacturing particular products may make sense for other reasons, even if there’s a net energy penalty. For example, it may reduce the burden on landfills, cut down on the need to dispose of toxic materials, or produce needed jobs. “We’re not saying you shouldn’t do it,” he says. They’re just suggesting that it’s worth understanding the full impact of the decision to remanufacture.

The study, coauthored by students of Gutowski’s and by Stephen Graves, a professor at Sloan and in the Department of Mechanical Engineering, was published in the journal Environmental Science & Technology. As Gutowski puts it, it “takes what appears to be a simple, straightforward problem and shows that the world is a far more complicated place than people thought.”