My Solar Bicycle

I am anticipating peoples’ reactions:

“Why do you have solar panels on your bicycle?”

“What do you do when it rains?”

“Can the solar panel drive the electric motor directly?”

“Did you make it yourself?”

“What the…”

Allow me to explain. My vehicle of choice is a “stoked Xtracycle”. (For those of you not “in the know”, an Xtracycle is a type of cargo bike that has an extra long frame. And “stoked” means that my bike has a Stokemonkey electric motor that helps me out on the hills.) In general this summer I’ve been biking 10 to 20 miles a day and then recharging my battery overnight by simply plugging it into an outlet. However, next week I’m going on a 3-day 240-mile camping trip through the Adirondacks where I might not have access to an outlet. The solar panels will help extend the range of my bicycle. So to answer your questions:

“Why do you have solar panels on your bicycle?”

I use them to extend the range of my electric cargo bike for long trips (plus they were fun to make). I will carry two batteries on my trip, each giving my bike a range of 20 to 40 miles. On a sunny day the solar panels can recharge one of the batteries while I am riding, adding an additional 20 to 40 miles for a total range of 60 to 120 miles a day. I anticipate some hills and I’ll be carrying a load, so a 60-mile range is probably more accurate. I may need to pedal the last few miles on some days.

“What do you do when it’s cloudy or it rains?”

I plan to stay in a hotel some of the time and recharge my batteries there.

“Can the solar panel drive the electric motor directly?”

Not really. The solar panels don’t produce enough electricity instantaneously. For example the solar panels only produce about 40 watts of power at a given moment, whereas my bicycle needs about 400 watts of power to go up a hill. The main purpose of the solar panels is to charge the battery over time.  Since charging happens slowly, 40 watts is enough to charge the battery. It takes roughly 10 hours of charging to store one to two hours’ worth of electrified riding time in the battery. And one to two hours of riding translates into 15 to 30 miles.

“Did you make it yourself?”

I already had the stoked Xtracycle, which is described on my About This Bike page. As you can read there, an electric cargo bike can be had for $1000 to $3500. And I had already constructed the canopy frame for a previous project, the Bike Wagon Canopy ($150). I found the canopy was somewhat wobbly with the weight of the solar panels so I had to strengthen it with guy wires. It remained for me to add the solar panels and the electronics. I used maritime-grade solar panels that were designed to keep sailboat starter batteries charged up, so they are extra-sturdy and consequently somewhat expensive. I’ve since seen panels with almost twice the power at 3/4 the price. Cost of panels: $900 to $1200. I am using three 12-volt panels in series to produce the 36 volts required by my battery. I spent a lot of time researching what sorts of electronics I would need between the panels and the battery, and finally concluded that I can just plug the panels into the battery directly. (I plan to write more about this in a later post.)

Total cost for a solar bicycle: $2050 to $4850. Not bad for a vehicle that can get you both out of the car and off the grid.

  • I get a lot of inquiries from people who want to stick a panel on the snapdeck and ride forever, “free,” and can I sell them the parts? Uh, no. OK, I’m exaggerating, but only a little. You’re living the dream!

  • What is your touring “weight?” Is the fully loaded weight so great that you need the supplementary power?

    • admin

      Here’s what I expect to be schlepping tomorrow:
      Unloaded stoked xtracycle: 83 lbs
      racks, canopy and solar panels: 8 lbs
      extra battery: 12 lbs
      camping gear, clothes: 20 lbs
      food: 10 lbs
      Water: 5 lbs
      Total: 138 lbs

      The solar panels don’t provide any extra carrying power–they won’t even be connected to the battery powering the motor, they’ll be charging a spare battery. The solar panels just increase my range.

  • Awesome!

  • Wow…. nice rig. I’m curious though… if you took the monkey off (along with the batts and associated hardware) would the weight savings make it possible for you to carry the touring load without need for the system in the first place? I’m just asking, not denigrating the system as I’m a Big Dummy/Stokemonkey owner myself, I’ve just never used it the way you are for powered loading TOURING, only for around town stuff where I only need 1 battery and can plug in when I need to. Todd Fahrner (owner/creator of stokemonkey) did a fairly long trip with his family (80-100 miles if memory serves) and was able to eke out quite a bit more miles from his system than I thought possible. I look forward to learning more about your system: what size batteries are you using? Are you using official Stokemonkey batteries or something of your own devising? Todd and I had several discussions a few years ago about using Lithium Ion or Lithium Polymer batts instead of NiMH, but with the new advances in LiFEPO4 batts, it seems to me that they might be a good alternative in this particular application. They are also a nice middle ground (at least to my cursory examination) in terms of price between NiMH and LiPO’s.

    • admin


      I did consider making the trip un-stoked. My bike would have weighed 90 pounds instead of 140. And it certainly would have saved me the trouble of looking for a place to charge my batteries every evening. But the reasons in favor of touring stoked are: the motor more than makes up for its weight so that hill are relatively easy; I could go farther (100 miles instead of the 60 or so I’d attempt unstoked); and I wanted to test the possibility of using solar power to supplement my batteries. I would also use the Stokemonkey if I were to carry a passenger such as my 10-year-old daughter.

      Todd increased the range of his bike by being very disciplined about his power use. I did the same on my trip by using this formula: if my speed fell below 10 mph I applied power up until my power use exceeded 400 watts. This enabled me to go 50 miles on a charge, and since I had two batteries I could go 100 miles a day. (The solar panel provided an additional 100 watt-hours a day.) My batteries are LiFePo4 from which are made for Stokemonkey:

      One thing I learned from my trip: even though the solar panel I had didn’t supply much of my total energy, it showed that the concept is sound. I’ve since calculated that it is entirely possible to make a bike-mounted solar panel powerful enough and practical enough for an electric-assist bike. A well-designed panel can supply power as fast as the rider uses the power. This is pretty exciting. Here’s what I posted recently to the roots radical Xtracycle Yahoo group:

      I have to admit that my first solar bike prototype is at best “almost not
      crazy”. It’s simply not powerful enough to justify the expense unless your goal
      is to learn about the technology and to inspire others. (So I met my goal :-).)
      I have high hopes for a second prototype though: a bike-mounted solar panel that
      can put out 100 watt-hours per hour. That would be producing about as much power
      as someone uses for moderate touring. Note that in Todd’s description of his
      epic stoked bike tour
      -back-again/) he said his goal was to use 8 watt-hours of power per mile. At 10
      mph that translates to 80 watt-hours per hour. So if Todd had had a 100
      watt-hours per hour solar panel he’d be good to go with power to spare. Groovy.

      My first prototype put out 25 watt-hours per hour. So I’d need to quadruple it’s
      output to meet my goal. This is entirely possible without any new technology.
      Using polycrystalline solar cells instead of flexible panels can add 100%, using
      a charge controller can add 30%, incorporating some sort of tracking mechanism
      can add 50%, and simply increasing the size of the panel can add another 100% if
      necessary. That’s potentially almost 8 times the power of prototype one.

      Prototype two may even be less expensive than prototype one (but require a lot
      more labor). This is because I’ll need to solder together 72 little
      polycrystalline solar cells, figure out how to mount them, and give them a
      protective cover. The flexible solar panels are a lot easier and sturdier but
      they are more expensive and less powerful.

  • Hey

    Nice work with the bike, how long would it take you to recharge the bike… say if you went on a ride and stopped at the beach to read a book.

    take care

    • admin

      With the setup I had for that trip it would take four to eight hours to recharge the battery. And that’s under ideal conditions of full sun and having the panels spread out and facing the sun. (Spreading the panels out and pointing them at the sun almost doubles the amount of power they produce.) I think it’s possible to build solar panels that can produce almost as much power as I use for moderate pedaling assistance. One improvement would be to use more efficient types of solar cells. Another would be to make larger solar panels. Still another would be to improve the electronics for charging the battery.

  • Cool! I’m another Big Dummy owner, not Stoked. What is the additional drag of the PV canopy?

    • admin

      The canopy definitely creates some wind resistance, but I rarely go fast enough for that to matter. It’s possible to set up the canopy so that it’s lower (just above the Xtracycle snap deck) by using fewer tent poles. There is less wind resistance in the lower position.

  • How about a looong trailer with a ton of panels? Could stash the cargo under the panels, and free up room for a passenger or two, while getting enough extra juice to still boost range and speed?

    • admin

      I’m looking forward to this summer so that I can try out some more of my solar bike ideas. I was inspired by the guys-in-a-garage at I have a BOB trailer that I plan to festoon with solar panels. I purchased a box of individual solar cells that I plan to solder together. The cells are twice as efficient per area as the flexible panels and they are about one tenth the price, but they are very delicate. So I’ll have to summon some engineering mojo to make a lightweight but sturdy enclosure. And ideally I should have some sort of tracking mechanism which can double the power output but again is an engineering challenge.

  • I really liked your idea with the solar panel.

    Maybe you would also like to share your experience with others in the interactive area of our website 🙂

    • admin

      I plan to work on it some more this summer so I’ll definitely be adding more.

  • we’re curiously waiting for it!

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  • syarifudin syarifudin

    very-very good idea. i want to make solar bycycle in Indonesia because my country everyday sunny. 

    • pavlo stouppos

      Hello from Greece, we have plenty of sun too. When you are ready let me know. I am interested for at least a personal bicycle but also for importing. Thanks.

  • vijay guruji

    thank,,,,,for good idea i want make solar bicycle if you have some new tips gate me.

  • sachin

    why do you have batteries in your bicycle?….

  • Remle Duff

    The question that immediately sprang to my mind on seeing your canopy was, “how does it handle the wind?”

    It seems to me that were it positioned low with it covering the trailer with no gap between the two that it would still get most of the sunlight, but would not present the extra cross-sectional area and what area it did present would be down lower where cross-winds would have less leverage.

    • Laurence Clarkberg

      Wind was not so much of a problem (though it was a problem with the Bike Wagon Canopy I made earlier). The solar panels are positioned high so that the Xtracycle deck can still be used to carry passengers. Perhaps an advanced design would allow the solar panels to be raised and lowered as needed.

  • Edward Vincent Boylan

    Hey Larry, I have a Radish cargo bicycle with a Bionx kit. How do you like the monkey? I was back and forth on the two designs for a while before I settled on the Bionx kit I got. My pitfalls so far have been that the system is DEFINITELY designed strictly as an ASSIST and not a motor. This is all well and good as it works by amplifying my pedal strokes with a thrust mode that can push it up to 20mph on flat terrain. But it will not work at a dead stop, you have to be rolling slightly and then it’ll kick on, annoying but not a killer. There are days when I’m exhausted (I ride roughly 40 miles a day to and from work, lunch, and my gym) and I wish I could just push the button and go but this system…i won’t say can’t but does not do that…well.