How Many Calories Do You Burn When Using a Human-Powered Generator?

 5 Nov 2019 -  Jim Gregory -  ~10 Minutes

One of the benefits of using the PedalPC is the ability to exercise while you work. Since a common goal of exercise is to control your weight, it’s natural to ask, “How many calories do you burn using a human-powered generator like the PedalPC?”

In this article, I’ll look at what factors influence your calorie expenditure when using a human-powered generator, the different ways to measure it, and how the PedalPC measures it.

The Three Factors that Determine your Calorie Burn

There are three factors that influence how much energy you expend when you use a pedal-powered generator (or, for that matter, any kind of stationary bike):

  1. how fast you pedal (i.e., your pedaling cadence),
  2. how hard you pedal (i.e., how much effort you apply to the pedals), and
  3. how long you pedal (i.e., the length of time you spend pedaling),

The number of calories you burn is directly proportional to each of these. If you increase the speed, force, and/or length of time at which you pedal, you’ll burn more calories.

The power required by the equipment you’re powering determines how hard and how fast you have to pedal. If your computer and associated equipment draw a lot of power, you’ll need to pedal faster and harder to keep it running.

How long you pedal is, of course, up to you. But if you are using a pedal-powered generator to recharge batteries, then the length of time you pedal will depend on the size, recharge rate, and depth of discharge of the batteries.

(Note that the time it takes to recharge a device like a phone or laptop is determined mostly by the device, not the power source. The recharge time will be the same whether it’s plugged into the wall or recharged using a generator. Pedalling faster or harder than required won’t shorten recharge time, either.)

I mention all this because, unlike other pieces of exercise equipment, your calorie expenditure using a pedal-powered computer is, in some ways, beyond your control. You can’t simply increase the resistance setting on the machine like you can on a stationary bike.

Instead, you have to manage your load. If you want to burn more calories, just add additional devices to your little “picogrid”. If powering your load is too much work for you, you either need to power fewer devices or choose ones that are more power-efficient.

Now that you know what determines how many calories you burn, let’s look at how you can measure it.

How Calorie Expenditure is Measured

It’s almost impossible to measure your calorie expenditure exactly. The best you’ll be able to do is get a really good estimate. Three methods can be used to do this.

Indirect Calorimetry

Indirect calorimetry measures the amount of oxygen you consume while you exercise. It’s the most accurate method.

The theory behind this method is that when you’re exercising aerobically, the primary source of fuel in your body is from the breakdown of fats and carbohyrdrates. Both require oxygen for that breakdown to occur. By measuring the rate of your oxygen consumption, you can calculate the rate at which that breakdown is occurring, and therefore how many calories you are burning over time.

The problem with this method is that the equipment is very expensive, and the mask you have to wear to measure your oxygen consumption can be uncomforable. (I wore one several years ago as a participant in an exercise study.) For this reason, this method is limited mostly to research experiments, where accuracy is important.

Heart Rate Monitor

The second method invoices measuring your heart rate. It’s based on the theory that the amount of calories you burn is proportional to your heart rate. The harder you exercise, the more your heart rate goes up, and the more calories you will burn.

This method is used by wristband and chestband fitness trackers made by Apple, Fitbit, Polar, and others. It’s simple, easy to do, and relatively inexpensive.

Unfortunately, multiple studies have found that while heart rate monitors measure heart rate fairly accurately, they do not accurately measure energy expenditure during stationary cycling. The mean error is at least 20% and often more than 60%.

Power Output

The third method of measuring your calorie expenditure involves measuring your power output. This is the method used by most types of exercise equipment, including most stationary bikes. It’s also the method I use.

It’s based on your Net Energy Efficiency (NEE), which measures how efficiently your body uses energy. NEE defined as:

NEE = power output / net power input

where the net power input is the amount of calories you expend per hour while exercising, over and above your body’s normal resting metabolic rate. This is the variable most people are interested in–how many extra calories you burn compared to just sitting still.

You can calculate the net number of calories you expend per hour by simply rearrranging the terms in this equation:

net rate of calorie expenditure = net power input = power output / NEE

Thus, if you can measure your power output while you’re pedaling and know (or estimate) your NEE, you can calculate how many calories you burn each hour.

Several studies have calculated the NEE using a stationary bike, so that variable is known. (We’ll get into that later.) The only variable left to measure is your power output.

Measuring Power Output

The power output of any piece of human-powered equipment, be it an exercise bike or a pedal-powered computer, can be measured in one of two ways.

One is using a power meter. This devices calculates your power output by measuring your cadence and torque applied to the cranks.

It’s worth noting that while most exercise bikes imply they have a power meter, few inexpensive stationary bikes or bike desks actually do. Some of the least-expensive bikes I have examined only measure your pedaling (cadence), but do not take into account how much force you’re applying to the pedals. As was mentioned at the top of this article, both measurements are needed to accurately measure your power expenditure.

Some models that use the resistance setting to estimate pedaling effort still lack accuracy. A 2014 study using a popular recumbent exercise bike found that the power displayed on its console was about twice the actual value.

The other method of measuring power output is to connect a generator to the cranks and measure it’s power output while driving an electrical load–which is exactly what a pedal-powered generator does. All that’s required is to measure the generator’s output and make adjustments to account for energy losses.

Estimating Calories from Generator Output

The efficiency of a human-powered generator is the ratio of its electrical power output to its human power input:

generator efficiency = electrical power output / human power input

This equation can be rearranged as:

human power input =  electrical power output / generator efficiency 

We can substitute this into the NEE equation above:

net rate of calorie expenditure = power output / NEE
        = ( electrical power output / generator efficiency ) / NEE

This shows three values are needed to calculate how rapidly you burn calories using a human-powered generator:

  1. Power output of the generator
  2. Estimated efficiency of the generator
  3. NEE

Here’s how to calculate each of them

Generator Power Output

A generator’s power output (measured in Watts) is the product of its voltage (in Volts) and its current output (in Amps):

P = V x A

Generator Efficiency

Generator efficiency is covered on another page on this site. I estimate the overall efficiency of the generator I use to be about 75%.

Net Energy Efficiency (NEE)

NEE varies somewhat from person to person, and increases with power output at low power levels.

Net Efficiency vs Power Output at Low Power Output Levels (data from M. Reger et al. (2013) Exercise efficiency of low power output cycling)
Power Output Net Efficiency
10 W 5.65 %
20 W 9.95 %
30 W 13.68 %
40 W 17.06 %
50 W 18.82 %
60 W 20.11 %
80 W 23.90 %
100 W 25.79 %

Studies have calculated the average NEE to be about 19% to 22% for healthy individuals at a power output of approximately 60W pedaling 60-90 RPM. The other ~80% is lost mostly as heat, and to other metabolic processes associated with increased movement.

Putting it All Together

As an example, I am currently generating 50 W of electricity while I write this. My generator’s efficiency is about 75%. My power input to the generator–the power my body is supplying–is:

power input = generator output / generator efficiency
                    = 50 W / 0.75 = 66.7W

In other words, I need to supply 66.7 W of power to generate 50 W of electricity using my generator

Assuming my body has a NEE of 20% when producing 66.7 W, my estimated calorie expenditure rate would be:

rate of calorie expenditure = power input / NEE
                        = 66.7 W / .2 = 333 W  = 287 kcal/hr

since 1 Watt is 0.86 kcal/hour. (A nutritional calorie is 1000 calories or 1 kcal. In the US, it’s often denoted with a capital “C” to distinguish from a thermal calorie).

By way of comparison, the estimated rate of calorie expenditure for a 160 lb bicyclist riding at a leisurely pace of < 10 mph is 292 kcal/hr.

Limitations

The values obtained by this method of power calculation are heavily dependent on the assumptions for NEE, and to a lesser extent generator losses. If my body actually has a NEE of 22%, for instance, my calorie expenditure would be 9% less (261 kcal/hr). If the generator’s actual efficiency is 78% efficient rather than 75%, my calorie expenditure would be 4% lower (276 kcal/hr).

Conclusions

The number of calories you expend using a pedal-powered computer depends primarily on how much electricity your computer and other equipment require. The more power they draw, the more calories you will expend.

Recharging batteries will determine how long you need to pedal. The larger the battery is and the greater it’s state of discharge, the longer you will need to pedal, and the more calories you will burn.

Your calorie expenditure also depends, to a lesser degree, on the efficiency of your generator system. A more-efficient system will transfer more of your energy to the equipment you’re powering, so you don’t have to pedal as long nor as hard.

Your metabolic efficiency also influences your calorie expenditure. If your body is especially good at converting food calories into work, you’ll expend fewer calories than others with a less-efficient metabolism, all other things being equal.

Producing 50 W of electricity results in a calorie expenditure of about 287 kcal/hr using a typical human-powered generator, or about 5.7 kcal per watt-hour. The actual values will depend on your personal metabolism and your generator system’s efficiency.