When I read Grist’s recent article about a stationary bicycle that could power “just about any gadget you might want to use around the house or farm,” my first thought was: How cool is that? I want one.

My second thought: The “dynapod,” as the machine is called, is one of the dumbest ideas for saving energy I’ve ever seen.

Built by Pedal Power Engineering, a small company in upstate New York that claims to be “committed to conserving our natural resources,” the dynapod is designed for off-the-grid and do-it-yourself types who churn their own butter and split their own firewood, but also make blender drinks and can’t live without their cell phones and computers. I’m still charging my laptop from the wall socket and feeling a bit guilty about that. But splitting logs? I have an app for that: a husband with a maul. And I can’t imagine why I’d need a machine to power a blade sharpener or coffee grinder when there are simple hand tools that do the job nicely and don’t take up much space in the kitchen. What’s next—a bicycle that charges your electric bicycle?

The dynapod appeals to conservation-minded people in the same way that wind turbines and biofuel-powered cars and hand-cranked radios do: They’re hard, shiny machines that purport to produce “free” energy. Not just free in terms of cost and independence (because you no longer have to pay a big, bad corporation for electricity or gasoline), but also free from guilt. Because you’re not polluting the air, right?

Unfortunately, things aren’t that simple. Before you even climb aboard your dynapod or generate your first watt of green energy, you’ve already stomped another carbon footprint into the sands of time through the manufacturing process. And until the energy embodied in machines, vehicles, buildings, gadgets, food, clothing, and other consumer purchases comes to be understood as part of total energy consumption, people can’t make well-reasoned choices about how to reduce their climate impact.

The life cycle of a bicycle. The bicycle may be the most energy-efficient machine ever invented. The calories that you burn to go a mile on a bicycle, if converted into a calorie-equivalent amount of gasoline, wouldn’t even move a car 100 feet. And it takes less human power to bicycle one mile than to walk that same distance. Similarly, pedaling a dynapod generates more horsepower than hand-cranking.

But while one machine may be more efficient than another, consumers should stop to consider the total energy use represented by each one. A full life-cycle assessment of a bicycle’s contribution to reducing climate impacts would have to consider the energy used for mining the raw materials, manufacturing the bicycle, transporting it, and—eventually—recycling or disposing of it. The assessment would also have to consider the bicycle’s usage and life expectancy: A bike that is ridden a few times and then abandoned in the garage will never save enough gasoline to “pay back” the energy invested to create it, but a bicycle that regularly replaces car trips can make a positive net contribution.

Life-cycle assessment has become an important tool for engineers to understand the true environmental costs of any given technology, but it’s mostly ignored by the public. When people shop for a car, they pay attention to the fuel efficiency rating but rarely consider how much energy went into making the car. People tend to be more interested in how many calories are in a hamburger than in how many calories it took to grow the meat. That’s too bad, because life-cycle assessments can help consumers make better energy choices.

Carbon debtors. When shopping for a new appliance, many people opt for the model with the lowest price. Some people are smart enough to know that it pays to buy an energy-efficient model, and a few people even take the trouble to calculate how long it will take to pay back the financial cost of buying or upgrading items such as kitchen appliances, solar panels, and new windows. Very few people, however, inquire about the carbon payback period for their purchases—in other words, how long and how frequently they will have to use a product before it has produced or saved more energy than it consumed. That’s not listed on Energy Star labels.

You don’t start “saving energy” on the first day that you install a new refrigerator, for example. Yes, it may use less energy than the one it replaced. But you now have two refrigerators on your carbon balance sheet instead of one. You have essentially taken out a carbon loan against the emissions reductions that you expect to achieve in the future (assuming you don’t keep your old fridge in service as a beer cooler in the garage, as many people do). This is a difficult concept for most consumers to grasp, because they aren’t involved in the front end of a product’s life cycle. When they take possession of the product, they mistakenly believe they are starting with a clean energy slate.

The same concept applies to new industrial power plants, whatever kind of energy—fossil fuel or renewable—they are producing. If you want to know how much a given technology will help or harm the climate, you need to know how much energy goes into building, operating, and decommissioning the plant. And you need to know something about the availability of the resource: How often and how hard does the wind blow at the wind project site, for example?

Life-cycle assessments sometimes produce surprising results. For example, scientists from the University of Aberdeen published an article in the journal Nature last September warning that constructing wind turbines on pristine Scottish peat soils gives the appearance of reducing carbon emissions but may actually do the opposite—because peat-covered land releases lots of carbon into the atmosphere when it is drained for construction.

Even a barrel of oil doesn’t have the same carbon footprint as it did in 1950, as two researchers at the University of Alberta observed in an article published in Science in March. Oil is less accessible than it once was, so it takes more energy to obtain it now—even without increases in consumption—and will take even more in the future.

Breaking even. The time it takes for a particular device, project, or industry to reach the breakeven point—where it has produced more energy than it has consumed—varies from technology to technology. Stanford University researchers recently calculated that there is a better-than 50-50 chance that the world’s existing solar photovoltaic panels last year generated more electricity than went into making new panels in the same year. The researchers predict that the industry will pay off its carbon debt in full by 2020. Only then will photovoltaic panels be a net benefit to society.

How do solar panels stack up against other technologies? A review of the scientific literature conducted by the Intergovernmental Panel on Climate Change in 2011 reported that hydroelectric power had the lowest life-cycle emissions, followed by wind and nuclear, with solar technologies trailing farther behind; natural gas and coal weren’t even close. More recently, a committee of nine experts on climate, energy, and economics who studied the United Kingdom’s carbon footprint published a report in April that found nuclear power had the lowest life-cycle emissions, followed closely by wind power. Solar photovoltaic emissions were estimated to be somewhat higher than for nuclear and wind, but not nearly as high as for fossil fuels—even when fossil fuels were coupled with carbon capture and storage processes, such as scrubbing carbon dioxide from power-plant flue gases and injecting it into geological formations. The report noted the difficulty of educating consumers about the concept of embedded carbon but said that there may be a benefit in labeling some products with their carbon footprint.

A well-educated consumer might think twice about buying a dynapod if it had a carbon-footprint label. While small-scale systems appeal to the environmentalist’s instincts for self-sufficiency, they are often less efficient—both in terms of energy and carbon—than larger systems. For example, in experimental results published a year ago by ConsumerReports.org, a home-scale wind turbine mounted on a roof for 15 months generated only enough electricity to power a window air conditioner for one afternoon. Recouping the financial cost of the turbine might take more than a lifetime, the report said. The carbon payback period would probably be of similar length.

Do it yourself. Carbon footprint labels would help consumers understand the real costs of their purchases. While a machine like the dynapod may make carbon sense if it’s used frequently by a small farm or factory, in most cases it probably doesn’t add up. Low-tech Magazine estimates that, using a pedaling machine, it would take two shifts of 10 people each going nonstop for a total of 16 hours a day to provide enough electricity for an average family in the United Kingdom, and twice that for an American family. And the flywheel, gears, and steel frame for the pedaling machine? I’m betting they weren’t mined and manufactured by a dynapod.

The dynapod does have at least one redeeming feature, though, pointed out by Philip Wilson in an online comment on the Grist piece: “The parts are visible so the design causes people to think and possibly learn about how machines work [and] how electricity is generated.” That, at least, it has done.