Generating electricity with a bicycle

Last update: January 2016

You can't generate a meaningful amount of electricity with a bicycle, and it won't save any money, either, because bike power generates such a tiny amount of electricity versus the cost of the setup.  And it might not even be green energy, once you consider the energy that's used to produce your fuel (food).

A typical bike generator can produce 100 watts.  If you pedal for an hour a day, 30 days a month, that's (30 x 100=) 3000 watt-hours, or 3 kWh.  That's less than 1% of what a typical family uses in a month (920 kWH).  You generated 0.3% of your energy, and continue to get 99.7% from the grid.  Good job.

But how much money did you save?  Well, since the average cost of U.S. electricity is 12¢/kWh, that one month of pedaling saved you $0.36.  Congratulations.  If the system cost $400, it would take only 93 years to pay for itself.

And that's before we consider the cost of food.  If you're overweight, like most Americans, then you can consider your biking energy "free" since you could be burning fat. Likewise, if you ride the exercycle instead of doing some other kind of exercise that you were going to do anyway, then the cost of your energy is also free.  But if you're not overweight and not exercycling instead of some other exercise, then you'll be buying more food to fuel your effort. Since it takes about 1 calorie to produce 1 watt-hour of electricity, your month of pedaling would require 3000 calories.  With the cheapest food you can buy, oil or flour, you're looking at $0.85 to create $0.36 of electricity.  So instead of saving money, it's costing you money to run your generator.  Other foods are even worse: Figure $5.41 for Cheerios, $6.15 for bananas, or $22.22 for Big Macs.

But money aside, isn't bicycle power a form of green energy? The answer is that it depends on where you get your calories.  Just as with the money costs, if you're overweight or exercycling instead of other exercise, then yes, the (piddling amount of) energy you create is indeed green.  But if you're already at a decent weight and not substituting for some other kind of exercise, then you're going to eat more food to power your effort, and the pollution caused to produce the food for your cycling is more than the pollution caused by geting the energy from the grid.  3000 extra calories from what a typical American eats will make 30 lbs. of CO₂e, or 15 lbs. for a vegan. By comparison, the energy from the power plant makes only about 5 lbs. of CO₂e to generate the same amount of electricity. (sources)

But there's more.  We've been assuming that 100% of your effort is captured as electricity, but that's not really true.  When you charge your battery, there's inefficiency in the friction drive between your bike wheel and the motor, inefficiency in the motor itself converting rotational motion into electricity, energy lost in the voltage regulator (which keeps you from putting too much energy into the battery too quickly), inefficiency of energy going into the battery, and then when you want to use the energy you made, there's the inefficiency of energy going out of the battery, and inefficiency in the inverter.  It's likely that at least a third of your effort will vanish into thin air. (more)

But what about putting the generators in gyms where people are exercycling or using ellipticals anyway?  Okay, let's take a look at the numbers: Texas State University put generators on 30 elliptical machines at a cost of $20,000. (source)  If we generously assume that each machine is used 1/3 of the time over a 12-hour period, that's 30 machines x 1/3 utilized x 12 hours x 100 wH/hr x 1 kWh/1000 wH x 360 days/year x 67% efficiency (generous) = 2900 kWh/year, which is not enough to power even one typical American house for the same period of time.  And cost-wise, the energy saved at 12¢ per kWh is worth $348 per year, so the payback time is close to 57 years, not counting maintenance or opportunity cost.  Yeah, it's green energy, because otherwise the exercisers' energy is wasted, but the far easier solution is to just stop using ridiculous amounts of energy in the first place. A single family can easily save more energy by making some modest changes than the entire fleet of ellipticals at TSU can produce.  The website you're reading now gives concrete examples of how to do so.

Resources

• Bicycle generators from Amazon (from $400, without battery)
  
• Complete system including bicycle stand, generator, and battery ($600)
• Free plans and video instructions for building a bicycle generator system
• "Bike powered electricity generators are not sustainable" - article from Low-tech Magazine