Rock The Bike

How many people will it take to power my laptop and projector, pedalboard, (or any device)?

Pamela Parker’s pedalboard at the Peace Day SF rally. Photo: Nio.

OK, so you have a device or set of devices in mind (like a laptop and projector) and you want to know whether it’s possible to power them with people on bike generators. Here are the steps to figure it out:

First, you need to measure the power consumption of the device you want to power. There are two ways to do this. You can read the writing on the product OR you can actually measure it using a device like the Kill A Watt. The second way is better for two reasons: It’s more educational and fun, and some product ratings are approximate. Some products use different amounts of power in different settings. For example, a loudspeaker will use way less power than its rating if you’re listening to music at moderate levels. So get yourself a Kill A Watt and get scientific!

Above: A Kill A Watt in use. Connect it to the wall, then connect your device(s) to it, and read the Wattage number. Careful to switch to Watt mode. The device may default to showing Volts. If the reading is very close to 110, then you are probably reading Volts (USA AC power voltage is 110). Look carefully and the Watt mode will have the units “Watt” next to the reading.

Next: Visualize what types of pedalers you are expecting at your event.  The Wattage you can expect per person will change depending on what types of people are pedaling! This is common sense: a competitive cyclist can generate more power than a 3rd grader.

Then use this table to get a suggestion of what Wattage output to expect per person.

Type of person Average Wattage per person
Competitive cyclists 150
Athletic college students 100
General public (adults only) 60
High school students 50
General public (mixed ages including kids) 40
3rd graders and younger 0-20

Once you have the numbers for your device and you know what kind of pedalers to expect at your event, divide the device’s Wattage requirement by the anticipated Wattage per person. The numbers above are based on our experience at hundreds of Pedal Power events.

Last thing to do is anticipate how many people will be at your event. In your mind’s eye, are there plenty of people available to pedal? Or will the crowds be more sparse? If you’re not expecting a big crowd, there may be times when your bikes don’t have riders. If this is the case, then each pedaler will have to work even harder to keep power ON. Or, you may experience an outage. If outages are not acceptable to you, then you may want to add more bikes, recruit some ‘ringer’ pedalers, or scale back on your demonstration (either picking a lower Wattage device or trying to keep it ON for a shorter period of time.)

Also, please note that people need a reward or feedback of some type in order to want to generate power to their fullest potential. The reward/feedback could be social (like pedaling in front of a group of people who are cheering you on), or experiential (pedaling to keep groovy music on), or it could be a dynamic element of the system itself, like a metric that changes when people pedal harder or softer. For example, the lights on our pedalometer rise and fall depending on how hard you pedal and how heavy the draw is.  If there is no reward, and the pedal power system doesn’t have any dynamic elements, people won’t pedal very hard or for very long.

Got it so far? Great! Now let’s do an example. Say you want to power up an LED projector and a laptop. Plug the laptop and the projector into a power strip, then plug that into the Kill A Watt and read the Wattage number. Let’s say that the answer is 180. Now, what type of people will be pedaling? Let’s say that you’re a High School science teacher, so you go to the table above, scan down to the row that says “High School Students” and use the number 50 for expected Wattage per person. Now divide 180 by 50 and you get a number between 3 and 4. To be on the safe side we recommend 4 generators to power the laptop and projector.

Here’s a tangible example: boiling water for tea!

These numbers assume an efficient Pedal Power system like the ones we sell at Rock The Bike. If your system is inefficient, it will feel harder for people to pedal the same amount of output Wattage. What makes a Pedal Power system efficient?

– The type of generator (hub or friction). We use mostly hub generators.

– The type of energy buffer (our Pedal Power Utility Box uses ultracapacitors)

– Bikes that fit the pedalers well. This includes raising and lowering the seat for every rider, as needed! Our Electric Fender Blender Pro can fit riders between 7 years old and full-size adults!

If you are using less efficient gear, or you don’t take care to fit each rider to the bike they’re pedaling, the numbers you get may be quite a bit lower, particularly for the athletic pedalers.