Human Power

Things to consider...

  Human Power and Electricity Generation.  For this discussion we need first to define a few things. Watts are a common metric for power both in athletes and electricity generation so that is convenient. Watts are a measurement of power output… they are not a “unit” of energy. For example a 60 watt bulb consumes power at a rate of 60 watts. If the bulb is on for an hour it has used 60 wH (watt-hours) of electricity. A wH is a unit of energy. We pay for our electricity in KwH (kilowatt-hours). That is 1000 watts flowing for one hour. Residential rates in the US range from about 6 to 15 cents per KwH. A toaster consumes about 1200 watts but only for about 3 minutes at a time. A cell phone about half a watt.  A Horsepower is 750 watts.  

Human Performance. Rough numbers below… there are many variables! A person cycling down a flat road at a “leisurely” speed would be putting out about 50 watts. An elite cyclist can average 350 watts for an hour or more!  An elite cycling sprinter can hit a peak of 1500 watts! But for just 10 seconds or so.  A 50 year old person that can average more than 100 watts for an hour is in pretty good shape. In cycling, weight versus power is important. Elite cyclists are usually about 160 lbs (men) as they want the lightest body possible with the fittest legs, lungs and heart. The true metric for cyclists is watts per Kilogram of weight at max power for 30 minutes.  So, let’s discuss potential fitness equipment electrical generation. Let’s say Sally can cycle at 100 watts for an hour. Her body would have made 100 wH (watt-hours). The best electrical generators are about 90% efficient but they are too expensive for fitness equipment. More common is 70%. Also there is the question of grid connection which has losses as well.  A fair estimate for grid connected machines would be 60%. So Sally could have made 60 wH electric during her hour on the bike. If electricity is 10 cents per KwH then she would have made .06 cents worth! Less than a penny! 60 watts is about what a full size laptop consumes.  So here is the crux: If it costs much to install equipment to capture electricity it is not worth it. It might take 20 years or more for the extra equipment to pay for itself. There is a positive learning aspect here though. When a person sees what it takes to make 60 wH they learn the value of electricity. But when I am asked about the “green” value here I caution people that one must consider not only the small amount of “clean energy”  but also what is the total energy required to create and dispose of the equipment.  BUT…. Almost all commercial cardio machines (ellipticals, climbers and bikes (non spin)) have generators already in them. They provide the adjustable and smooth resistance of the machine. When you press a button and it gets harder the generator is putting out more power which is fed to a heater coil and wasted as heat. Unless you are trying to heat a building in the winter this is wasted. Treadmills have a large motor to keep the belt moving smoothly. Treadmills consume 1000 plus watts. It would take 10 average cyclists to power one treadmill!   It is possible to “re-engineer” the electronics and grid tie machines with clever engineering. It is estimated that a grid tie machine could cost about $100 more than a standard machine. So, in this evaluation, grid tied machines probably make sense.  A good goal are machines that cost the same as standard machines, are fully recyclable and that don’t have a lot of extra nonsense on them like individual TV’s.