How Much Electricity Can A Human Generate Per Day?
One of the first posts on this blog answered the question how much electricity can a human generate. That answer, in retrospect, was incomplete. It calculated only the electrical power a person could generate, not the electrical energy.
Energy is the product of power and time. It is a more accurate measure of a human-powered generator's feasibility, because it takes into account both how hard and how long you have to pedal to power a device or appliance.
So this post again answers the same question, but this time from an energy perspective. It also considers how you could use this electricity plus the additional equipment you'd need to use it.
Beginning Assumptions
Let's first assume you have a pedal-powered generator that produces direct-current (DC) electricity. The generator may be a converted exercise bike, a modified bike trainer, or a specialized computer desk like mine.
Let's also assume the generator is connected to a battery. The battery is important for two reasons.
First, it acts as a buffer between your generator and your load. The voltage produced by a pedal generator can fluctuate wildly up and down, which can cause problems with many electrical devices. The battery helps to smooth out those fluctuations.
Second, the battery can store electricity while you pedal, which you can use to power equipment later when you aren't. This is important, as you'll see later.
How Much Electricity Can You Generate?
As I suggested above, your total electrical energy output each day is the product of your average power output and the amount of time you pedal:
E = P x t
where:
E = total daily energy output
P = average power output
t = time spent pedaling per day
This makes intuitive sense--the harder and longer you pedal, the more electricity you will produce each day.
Your power output depends on several factors, including your age, fitness, and levels of concentration and fatigue. Over a prolonged period, most people can continuously generate 30 to 75 watts (W) of electricity. Mine fluctuates from 70 W in the morning to 50 W in the evening. My overall average is about 55 W.
The length of time you can pedal each day depends both on your physical stamina and your other time obligations. Note that pedaling not be time-consuming activity or burdensome chore. You can pedal while engaging in otherwise passive activities like watching TV, playing video games, or working at your computer. This not only makes pedaling more enjoyable, it greatly increases the potential time you can pedal each day.
You don't need to pedal the full amount in one sitting, either. Breaking it up into 2 to 4 sessions per day not only reduces fatigue, it allows you to use a smaller storage battery, too, if the stored energy is used to power equipment between sessions.
I pedal whenever I work at my computer, which I typically do 3-4 times per day. This allows me to pedal around 4 hours a day without interfering with other daily activities.
Knowing (or estimating) these two values allows you to calculate how much electricity you can generate each day. For me, this is about 220 watt-hours (Wh):
E = P x t = 55 W x 4 hours = 220 Wh
If you power equipment like a TV or computer while you pedal, you'll need to subtract the power these use, since they consume a portion of your electrical output. I normally power my computer, monitor, and a small cooling fan, which together draw about 10 W. This reduces my net power output to 45 W and net amount of usable electricity to 180 Wh:
P = 55 W - 10 W = 45 W
E = P x t = 45 W x 4 hours = 180 Wh
Pedal-Powered Load Types
Now that you've estimated how much electricity you can generate each day, you can determine what you can do with it.
There are three categories of pedal-powered generator loads: impossible, doable, and practical. Impossible loads consume more energy over a day than you--or anyone else, for that matter--could deliver. Most large appliances like refrigerators and air conditioners fall into this category.
For instance, the most energy-efficient 18 cu ft refrigerator uses at least 358 kWh/yr, or 980 Wh per day. This is more than 5X the amount of electricity I typically generate. If your net power output is the same as mine (45 W), you would need to pedal almost 22 hours every day to power a refrigerator.
t = E / P = 980 Wh / 45 W = 21.8 hours
That is obviously not feasible.
Doable loads
Doable loads draw more power than you can produce using a pedal generator but consume only a fraction of you daily energy output. These include small appliances like toasters, small microwave ovens, and coffee makers. For instance, a 700 W toaster that makes toast in 3 minutes requires 35 Wh of energy:
E = P x t = 700 W x 3 minutes x (1 hour / 60 minutes) = 35 Wh
That's only 1/5 the 180 Wh surplus I produce each day. However, the power required (700 W) is 10 times more than I am able to sustain.
Doable loads like this require an intermediate storage battery. You pedal for several minutes until the battery stores enough energy to power the load. Then, the battery discharges the accumulated energy at a high rate to power the load.
Many batteries can't handle discharging 700W, however. The power transferred in an electrical circuit is the product of voltage (V) and current (A). Most pedal-powered generators use a single, standard 12 V storage battery. If such a battery were used to power a 700 W toaster, the current from the battery would be over 50 amps:
P = V x A
A = P / V = 700 W / 12 V = 58.3 A
This is a very high current, more than three or four times higher than a standard 110 V household circuit. A current this large requires both a large battery and heavy-gauge cables.
The maximum discharge current capacity of a lithium-iron-phosphate (LiFePO4) storage battery is normally twice the value of it's storage capacity, so a ~60 A load would require a 30 Ah battery. That's 3 times larger than the battery I currently use on my machine.
A better solution would be to wire multiple 12 V batteries in series. For example, four 12 V batteries in series would have a nominal voltage of 48 V, reducing the current drawn from the batteries from 58.3 A to 14.6 A. Their charging voltage now, however, is 48 V, well beyond the output of a typical 12 V pedal generator. There are ways to overcome this (e.g., using a boost converter, redesigning the generator so it turns at a much higher speed, or charging the batteries in parallel and discharging them in series), but they all add cost and complexity.
In addition to the storage battery issues, most small appliances also require a high-power inverter to convert the battery's DC output to the alternating current (AC) small appliances require. Often the battery and inverter are combined together into a device called a power station. Whether combined with the battery or not, a high-power inverter only adds to the system cost.
How long you need to pedal to drive a doable load depends on your average power output and how much energy the doable task requires. A person generating 70 W would need to pedal at least 30 minutes to make toast. Baking a potato for six minutes in a microwave oven would require pedaling over an hour.
I don't bother trying to power doable loads. The extra equipment cost and pedaling time isn't worth it to me. These loads are usually discretionary (e.g., you can choose to eat cold cereal instead of toast) and are better powered by solar panels or a wind turbine.
Practical Loads
Practical loads consume less power than you can generate. These include many laptops, small computers, modems, routers, and small TVs and monitors. It also includes recharging the batteries in phones, flashlights, and battery-powered indoor lights.
You can usually power multiple practical loads at once. As I write this, I'm currently powering a small computer and monitor, desk lamp, small battery charger, an internet adapter, and web server, as well as recharging my phone.
Practical loads are often powered directly from DC (i.e., they don't require an AC inverter). This saves the power losses of a DC-to-AC conversion. Many loads (e.g., USB-powered devices) need a DC-to-DC adapter to convert the voltage of the generator/battery to the voltage the device requires, but this is usually very efficient.
Like doable loads, practical loads can be powered by a battery when you are not pedaling; however, their current draw is far lower, requiring a smaller, less expensive storage battery.
Another difference between practical and doable loads is the pedaling time is less than a device's "on" time, not more. I mentioned above that pedaling at 70 W output for 30 minutes would only power a toaster for 3 minutes. Pedaling at the same output for the same 30 minutes will power a small, 1.5 W web server almost a full day [1]:
t = E / P = 70 W x 0.5 hour / 1.5 W = 23.3 hours
Conclusion
How much electricity you can generate with a pedal-powered generator depends on how much power you can generate and how long you pedal each day. It's possible to generate well over 200 Wh per day if you pedal while doing normal daily activities (e.g., while working at your computer). Generating electricity during these times not only increases the energy you can generate, it makes the task less boring.
Some loads consume more energy over a day than you can possibly generate. These obviously can't be powered using a pedal-powered generator. Loads that consume less energy but more power than you can generate are doable, but require more expensive equipment and pedaling time. Loads that use less power than you can generate are the most practical. They require a smaller, less expensive storage battery and less pedaling time. You can often power multiple such loads simultaneously.
[1] - This doesn't take into account the round-trip storage efficiency of the battery. If a 95% efficiency LiFePO4 battery is used, the actual runtime for the web server would be:
t = E / P = 70 W x 0.5 hour x 0.95 / 1.5 W) = 22.2 hours