# Exercise for Energy

## Elliot Hawkes November 16, 2013

### Submitted as coursework for PH240, Stanford University, Fall 2013

 Fig. 1: Could the energy produced by exercise make a significant contribution to the power required by a gym? (Source: Wikimedia Commons)

Two years ago, Time Magazine reported on a new form of gym: one that is powered by the exercisers using it. [1] Since then, the Green Microgym in Portland has thrived, and continues to attract customers. [2] The owner of the gym, Adam Boesel, makes claims, like 36% of the power needed for the gym comes from users and solar, and the gym requires 85% less electricity and has a 90% smaller carbon footprint per square foot than a traditional gym. [2] These are impressive claims, but before every gym switches over to machines that "plug out," the claims should be evaluated in more detail.

First, it is necessary to calculate how much energy a person can produce during exercise. If an average workout is around 20 minutes of actual activity, and at an average rate of output of 100W, yields an output of energy of 33.3Wh. This is quite close to the reported number of 37.5 Wh for the gym. [1] Since power costs 10.6 cents per kilowatt-hour in Oregon, and the output is .0375 kWh, the power produced by the gym user is worth about one third of a penny. [3] With a total membership of 200, if half of the people came to the gym everyday, this saves \$0.30 a day. At this rate, it does not seem as though the machines are worth it, but the previous claims were much more impressive. How is this rationalized?

The first claim is that 36% of the power comes from the combination of solar and the exercise machines. A standard residential solar panel from Ikea produces 3.36 kW. [4] Thus if the gym has a single solar panel and 20 people pedaling at the same time, less than 2% of the solar-plus-human power will be coming from the exercise, meaning about 0.7% of the total power consumption of the gym was fueled by humans. A second estimate agrees on the order of magnitude: Boesel reports that the combination of solar and human power saved him 37,000 kWh in 2009. [5] Estimating that half of the members do a workout in a day, all producing 37.5 Wh per workout, we get 100 workouts a day, or 3.75 kWh. Over the course of 52 weeks, running 7 days a week, we would get 1400 kWh over the course of the year. Thus of the 37,000 kWh produced by solar panels and humans, and the humans contributed around 3.8%, or just over 1% of the gym's total power consumption. To say that this stat of 36% is misleading is an understatement.

The other facts are impressive, but again somewhat misleading. While requiring 85% less electricity than a regular gym is a worthwhile goal, we can see from our previous calculation that a negligible percent is coming from the human power. The gym focuses on low power consumption, avoiding large, flat screen TVs, and from our numbers, can see that these types of changes are much more meaningful than "plugging out."

There may be one redeeming reason to convert human power to electricity: it gives us an idea of just how much energy we are consuming. Thirty minutes of sweat and hard work is worth just a third of a penny of power plant energy.

© Elliot Hawkes. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.

## References

[1] T. Newcomb, "In the Gym: Clean Energy from Muscle Power," Time, 26 Nov 10.

[2] K. Duncan, "Four Fitness Startups Find a Unique Niche," Entrepreneur, 27 Jul 12.

[3] "Electric Power Monthly," U.S. Energy Information Administration, October 2013, Table 5.6.A.

[4] M. Rising, "Ikea to Start Selling Residential Solar Panels," Detroit News, 30 Sep 13.

[5] M. Park, "Pedal Power is Fueling Green Awareness," CNN, 22 Apr 10.