This week, we had a reader write in:
Why has no one developed a battery that can be attached to a recumbent bike to gather energy when someone is pedaling? Thousands of hours of manual work is being wasted (not counting the health benefits)
The short answer is “conservation of energy”—if you’re putting work into charging a battery, it’s going to make it harder to get where you’re going by pedaling. However, some people have tried creating an electric bike with a motor and regenerative brakes, which allow the rider to simultaneously charge up and slow down. Unfortunately, converting between different forms of energy—like going from the mechanical energy of momentum to the electrochemical energy in a battery—is never a perfectly efficient process. Usually when you’re going down a hill, you’re going to have to go back up one of equal size pretty shortly afterward, so for most people (depending on where you live, of course) there aren’t that many circumstances where it makes sense to take the hit associated with storing the energy—often close to a 50% loss—rather than just maintaining it as speed.
But we get similar questions all the time asking about using humanpower to generate electricity, so this is a great opportunity to talk about the concept in general and get a feel for how worthwhile the process would be.

That dystopian future where everyone pedals for a living: Ethically questionable, sure, but what about economically? Image Credit: Black Mirror, Channel 4 
For starters, a joule is a unit of energy, and a watt is a unit of power—energy per unit of time—equal to one joule per second. Now there’s a nifty shortcut to get a feel for how much power something draws in terms of the physical work you’d have to do to produce that much power:
It takes about ten joules of energy to lift a onekilogram weight by one meter. Imagine holding a 2liter bottle full of water (which weighs exactly 2 kg) and doing a bicep curl, which involves lifting the weight by about half a meter for a full grown adult. That’s 10 joules—or 9.81, to be precise. If that number looks familiar, it’s because it’s the gravitational acceleration at Earth’s surface; since we’re working in metric units, things tend to be tidy like that.
Now, a standard incandescent lightbulb is sixty watts, so you’d have to do about six of those twoliter bicep curls every second (or one, with six times the weight) to put out enough energy to keep a lightbulb going. LEDs, which are much more efficient, can produce the same amount of illumination with ten watts—only two liters of water, but still one bicep curl a second.
But how much is ten joules compared to, say, the contents of a cell phone battery? Mine has a capacity of 6.85 watthours, equivalent to 24.66 kilojoules. To charge my cell phone battery, I’d have to do almost 2,500 of those twoliter bicep curls!
But arms are weak compared to our legs, which the reader’s question was originally about. So how much energy do we sink into a stationary bike? It’s not as clearcut as with lifting weights, unfortunately—it depends on everything from the rider’s weight to the resistance level they’re using, so it’s harder to do the math from first principles the way we did above.
In an ideal world, we’d be able to simply look at the number of calories burned by using a stationary bike for an hour—roughly 500—and convert that directly to joules, since they’re both units of energy. Unfortunately, many of the calories burned in aerobic exercise like this turn into heat, rather than usable energy, so just looking at a calorie count would overestimate the useful power output of a person.
Instead, we can look at another favorite piece of gym equipment: the stair climber. Let’s say we’ve got an averagesized man, 70 kilograms, climbing at the moderately strenuous pace of one meter every two seconds. Every meter climbed is about 700 joules, which means we’re putting out 350 per second, or 350 watts of harnessable power. Assuming you could capture 100% of this energy and put it into charging a battery, it’d take roughly 70 seconds of climbing to go from no charge to full on the phone. During that time, you’d climb the equivalent of 40 meters, or 120 vertical feet—about 12 stories of stairs (more if you’re smaller than 155 lbs, less if you’re heavier).
It’s not a trivial amount of work, even to power one of the lowestenergy appliances out there! Your average microwave oven, one of the highest, draws 1200 watts of power, meaning you’d have to climb for about four minutes to get a minute of microwave time. At the rate and weight discussed above, you’d get a little over 1.1 million joules from an hour of climbtime.
So how’s this compare to the way we currently get our energy? On average in the US, a kilowatthour (3.6 million joules!) of electricity from the power grid costs 12 cents. So let’s do a little math to figure out how much you could make as a professional powergenerator in our fossilfueled economy.
The Empire State building is 1250 feet tall (a bit more if you’re measuring to the tip of the antenna, but let’s talk climbable height). That comes out to 381 meters. For the average man, climbing the equivalent of 381 meters would generate:
Comparing that to the amount of “juice” you get for 12 cents, the future doesn’t seem too promising for anyone aspiring to be a professional stairclimber. Just how grim is it? A little more math reveals:
Sitting at a rowing machine and listening to the wheel spin away, you might think that any salvage would be better than nothing, especially given the sheer number of people working out at any given time. But an exercise machine that harnesses or stores the power you put into it would have to contain an array of magnets and metal coils, making it significantly heavier and more expensive to manufacture. At the current going rate of electricity, the cost of building all these extra components into your stair climber just wouldn’t pay off over the lifetime of the machine.
Hopefully, you’re walking away from all this with a better intuitive understanding of the scale of power consumption that goes on in the developed world. The ability to “eyeball” a wattage rating in terms of the amount of physical work it’d take might not be the most useful of skills, but it’s a neat trick and enables you to play a mental game of “could I handpower this if I really needed to?”
Leaving a lightbulb on when you’re not using it might seem like a trivial expense, barely worth going back to flip the switch—after all, it’s less than a penny an hour—but when you think of it in terms of the actual amount of energy that’s going to waste, and what it feels like to exert that energy for yourself, it can put flipping that switch in a new light…no pun intended.
—Stephen Skolnick