Sunday, August 16, 2009

How to build a solar powered bike

The original version of this "how to" article, written about a year ago, included gems like "save up money to live on for six months" and "quit your job." While this advice may be applicable to the no-compromise high-performance long-range solar bike project I'm still working on, it's pretty useless to the novice who is just starting to think about how to "add some solar" to their electric bike. Maybe you don't even have a bike but the thought of having a solar-charged electric assist sounds like a good idea and you just need some tips on where to get started and how much such a thing might cost. This article is for you.

The recumbent bike with the yellow tailbox you see pictured to the left is still my main focus but I've recently completed a second bike to use when the recumbent is disassembled for repair or upgrades. The new bike is a traditional upright mountain bike with an Xtracycle conversion kit. I've installed a hub motor and used batteries I had lying around from an earlier project. The photos show some proposed solar panel mounting locations. This baby is perfect for hauling groceries and large packages. With a couple of optional accessories it can even haul a ladder or a surfboard.

The middle column below shows exactly what I built. The left and right columns are suggestions for how to build a less expensive version or a deluxe version to match your budget.


Budget Version


My Version


Deluxe Version
Electric bike conversion
~ US$ 500~ US$ 1,300~ US$ 1,800
Bike $0 if you convert an older bike you already own or get donated for this project. A mountain or hybrid bike with 7 speeds or less in the rear is ideal but with sufficient determination and ingenuity almost any bicycle can be pressed into service. $325 New Raleigh Mojave 3.0 mountain bike. I considered converting my trusty Surly Long Haul Trucker for this project but it had the wrong kind of handlebars (throttle wouldn't fit), wrong size cassette (8 speed instead of 7 max needed for the hub motor) and 700c wheels instead of 26" I wanted for good hill-climbing torque. I also considered a used bike on Craigslist but concerns about getting a stolen bike and time and money spent getting a lemon into shape lead me to a new entry-level mountain bike from my local bike shop. $600 for a new mountain/hybrid bike with better quality components. Don't get a rear suspension if you're using the Xtracycle conversion and don't spend more than $600 to get a lighter bike since you're going to be adding motor and batteries which will make it heavy no matter how light the frame. Do spring for disc brakes because this baby will have a top speed over 30 mph and will weigh over 60 lbs plus rider. Sooner or later you will need to make an emergency stop and you will be glad you have disc brakes when that moment arrives.
Motor $350 bargain kit on eBay including hub motor, wheel, motor controller, throttle, wiring accessories (compared with middle option: less power, lower top speed, less durable, no watt-hour meter to measure remaining range in batteries). See also Amped Bikes $690 for 600W geared BMC V2-S hub motor kit including motor controller, throttle and Cycle Analyst watt-hour meter $800 for 600W geared BMC V2-S hub motor kit including motor controller, throttle and Cycle Analyst watt-hour meter. This is price is my best guess. You may find a better deal.
Batteries $100 36V 10Ah SLA (sealed lead acid) battery pack plus charger. Heavy and don't last long but they're cheap and recyclable. $250 approx value of used LiFePO4 48V 10Ah battery I had from a previous project (bought new on eBay 2 years ago for $600) $400 LiFePO4 48V 20Ah battery pack on eBay (August 2009 pricing)
Battery range Up to 20 miles without pedalling (varies greatly by rider weight, speed and terrain) Up to 30 miles with no hills, no pedalling, few stops, keeping speed under 15 mphUp to 60 miles with no hills, no pedalling, few stops, keeping speed under 15 mph
Top speed 15-20 mph 34+ mph (California legal limit is 20 mph) up to 40 mph with smooth high-pressure tires, flat, smooth road and rider weight under 180 lbs
  Solar conversion
 ~ US$ 200~ US$ 850 ~ US$ 1,700
Mounting rack $100 for a heavy duty bike rack for mounting your batteries and solar panels. It should be rated for at least 50 lbs (~25 kg). $450 Xtracycle FreeRadical/Longtail kit affords plenty of space for mounting 2 or 3 solar panels. I'm still working out the details of how to attach the panels. Check back in a few weeks for updates. $750 Xtracycle conversion including FreeRadical/Longtail kit plus KickBack stand, WideLoader and LongLoader accessories for carrying passengers and large loads with greater ease
Solar panels $100 for suggested minimum 30 watts at $3/watt on eBay. Good for 10-15 solar miles/day in the summer. Selecting solar panels for your bike.$300 approximate value of prototype solar panels I acquired under special circumstances (I work in the solar industry) $777 for three 24 watt SunWize Sol-Charger modules for a solar range of 30+ miles/day in the summer
Charge controller Not needed if you use SLA batteries and several small solar panels wired in series and facing the same way. For example three 12V 10 watt solar panels can be wired in series to get 36V and connected directly to a 36V SLA battery pack. It's not ideal but it works and it's cheaper than getting a charge controller. $100 Solar Converters CV 12/24-2PV solar charge controller. Takes nominal 12V solar input (up to about 20V) and delivers optimal charging voltage for 24V SLA battery pack (CC/CV, tapering off to 28.8V). Limited to 2 amps. This isn't ideal for LiFePO4 batteries but it seems to work just fine. $200 Solar Converters 5 amp 12V input, 36V output charge controller. They have a model with an adjustable output voltage which may be a better choice for your expensive LiFePO4 battery pack.
  Not included: Mounting hardware for solar panels and batteries.


How to do it even cheaper?

Walmart and Target sell electric bikes starting at US$300. That price includes the bike, motor and battery. If you are considering a bike in this price range think about the compromises in quality that had to be made to get the price so low.

If you think my budget version above is still too expensive and you have access to junk yard parts and a well-equipped workshop, you may consider building your own electric bike from scratch. Using old bicycle parts and almost any 200 to 1000 watt DC motor, a moderately skilled Jack or Jane of all trades with ample determination can build an electric bike on a shoestring budget. Batteries can be re-purposed from just about any source - old cordless tools, backup power supplies for computers, slightly used SLA's from medical equipment, even old car batteries may be enough to get you started and can fuel your enthusiasm until you save up for a proper LiFePO4 battery pack and charger.

The only item on the list that doesn't have a free or cheap alternative are the solar panels. I don't know of any sources for used solar panels. Here in California, they are phasing out the old emergency roadside call boxes so maybe you can find if the old solar panels from these call boxes are being sold at auction somewhere? Searching for used solar panels on eBay yielded a handful of scam auctions trying to sell used panels for $5 or more per watt. Keep in mind that you can get new panels on eBay for less than $3 per watt.


Original post (April 2008)

  1. Save up enough money to live on for at least 6 months. It is important that you do this before you proceed to step 2.
  2. Quit your job or arrange to take an extended sabbatical leave. You won't have time for your job once this project gets under way. You did follow step 1 first, right?
  3. Next, you'll need to decide between an old fashioned safety bicycle and a recumbent bicycle.
  4. The safety bicycle was invented in 1885 by John Starley and hasn't changed much since. The "safety" part refers to the improvement it represented when compared with the penny-farthing bicycles that preceded it. It doesn't offer much opportunity to add large solar panels so you'll be limited to a 20-30 watt panel or about 10-15 solar miles per day of charging in the sun. You may be able to bring that figure up to 50-75 watts if you add a trailer. The advantage of this approach is that you can leave the trailer at home on cloudy days.

    The recumbent holds every world record for human powered travel in terms of distance and speed (over 80 mph) thanks to it's efficient design. More importantly, it will allow you to mount 100-150 watts of solar panels directly on the bike. Based on my road tests, this would give you 40-60 solar miles per day in the summer. If you design your panels as front and rear fairings, you can actually improve the aerodynamics of your bike and help offset some of the added weight.

  5. Select a motor and battery. I use the Electro-Portal E-4 kit but there are lots of other options available. You will need a battery to store the energy produced by your panels when you're not using the motor, such as when you're stopped or when you're going downhill or just pedaling on a straight stretch of road. The battery will also provide the extra bursts of power you'll want when you're climbing a hill or starting from a dead stop. Start with an inexpensive SLA (sealed lead-acid) battery and upgrade to a lighter, longer-lasting LiFePO4 battery later on. The initial cost is higher but they last longer so the life-cycle cost is the same or even lower.
  6. You're going to have to design and build your own panels. Standard rooftop solar modules are designed to withstand impact by 1 inch hail at 50 mph, not to mention 30+ years of wind, sun and rain. They have to be very tough and durable to survive the extreme environments in which they're used. This is accomplished by framing the module with tempered glass and aluminum. These materials are heavy and don't belong on a bicycle. You may also be tempted by light, flexible thin-film solar modules. They can be rolled up and are virtually unbreakable but they suffer from a low efficiency, meaning your panel would need to be twice as big to get the same amount of power. Since you want to get the most power possible from the smallest panel possible, you're going to need to build your own panel from scratch. Given enough time and patience, anyone can build a solar panel. I didn't know anything about solar power when I started. You should start by purchasing this DIY solar guide from Stephan Hughes. Stephan is a great resource for do-it-yourselfers. [Update: May 20, 2008 - Stephan's eBay store appears to be offline. My email asking about this change has gone unanswered. Contact me via the comment link below if you're looking for resources for building your own panel.]
  7. Start a blog and tell the whole world about your crazy adventure.


Laara said...

Wow. You are brilliant! I began researching tonight and your site offers a perfect alternative to gasoline driven vehicles!! I hope to convert to something with 3 wheels and I would have my son build it and maybe one for him also. I'm bookmarking your site so that he can have a look. (I am a 63 year old Canadian lady!) Let's just do it! Awesome.

Tomas said...


You've done an incredible job on your bike !!!I am trying to build a 2 person solar trike, would you recommend any specific brand of high efficiency solar panels (cloe to 30%).

Thanks a lot !!

Mark said...

Hey Tomas. I did a lot of searching for high efficiency panels about a year ago. At that time, the only option I found with a 30% efficiency were triple-junction cells made by Spectrolab (their website is a disaster). These are intended for aerospace use like satellites and such and cost about US$300 per cell. A 50 watt module made from such cells would cost over $20,000. I don't have that kind of budget. The next best option I know of are SunPower modules with about 22% efficiency. They sell for $3-4 per watt but I don't know where you can buy just one or two modules. If you find a better option, please let me know so I can share with others.

santiago said...

Hello mark..

My name is santiago and I`m from Cordoba Argentina. I`ve been reading all your posts and looking for other related information. I`m really interested in building a solar bike but here costs and material disponibility is almost none. For example buying a nacional (arg) made solar panel that produces 60w it`s about 500 U$S. My idea its a tricicle ( or Trike) in which i can travel about 200km per day.. And who says i would probably be in USA for one of those solar races there organizaed.. big dreaming is good hahaha..

I`m planing to made my own solar panels buying solar cells in USA by eBay..

My e-mail is I hope we can share information..

And Congratultions for your bike..

PD: sorry about my english.. haha


enso said...

What is the source you mention for the solar charge controller? this is a great project I am thinking of building a trike like your V 1.0 any reason you moved away from the trike design to a 2 wheeler?

Mark said...

Hi enso,

Here's where I got my charger controller. This seller keeps many models in stock and is based in California.

The company that makes these is in Canada ( and they can direct you to a reseller closer to where you live.

I went with a 2 wheeler because I started commuting 20 miles a day in traffic and felt I needed to be higher off the ground for visibility. I also wanted a full suspension to deal with the many unavoidable pot holes and curbs along my commute. I put 3000 miles on my trike before switching and was quite happy with it. The 2-wheeler just fits my current situation better.


Morgan said...

Here are some photos of my first solar bike.

mohit said...
This comment has been removed by a blog administrator.
BeetleDozier said...

I would like the further information on building your own solar panel.


shweeto said...

Greetings to the author,

This is an incredible work done by the author and very informative as well. Congratulations! on your success. I am Nidhi Batura from India. I am working with SolarIndiaOnline as head corporate communication, this is the first media website for solar in India. We have a database of more than 35000 + people. I was reading your article and can see it is very informative. We publish online newsletter and magazine on solar in our website in which we require new research like this. I think if you can send me an article on any of your new research whether on solar powered bikes or any thing else, that would be highly appreciated and published under your name to all our data base and our newsletter and magazine. We publish this newsletter and magazine every month.
If you have anything in this context you can mail me back on,
You can also check our site i.e.
Hope to hear from you soon.

Nidhi Batura

Anonymous said...

what are your calculations for using a 200-1000W motor. the air resistance and mechanical resistance for 200 lbs is only about 300W. can you post the final statistics for your bikes, hopefully including dimensions of the bike and solar cells?

Mark said...

"200-1000W" is based on my initial research of various electric assist bicycle forums and my own practical experience riding several different electric assists totaling 10,000 miles over the last 3 years.

The 200W low end is the legal maximum in some countries (Australia, I think) and is the bare minimum in my opinion. Over 1000W starts to get into speeds that feel unsafe on a bicycle. The frame, rims, shocks and brakes just aren't designed to hit a pothole if you're going over 40 mph.

My typical commute involves cruising at 25 mph on flat ground while the motor contributes around 400-500W and I'm pedaling with a moderate effort.

Bike plus rider plus luggage is around 250 lbs., 60-70 psi 1.5" tires, 20" front, 26" rear. Short wheel base recumbent bike. You'll need to adjust these numbers if your speed, weight, motor/gearing efficiency, aerodynamic drag or terrain is different from mine.

Mark said...

Here's a post I wrote about selecting solar panels for a bicycle.

segue to solar said...

This is a fantastic idea... the solar bike. Is anyone manufacturing these?

We would like to buy one, NOW!
mark and sonia

kjcad said...

I just bought a used pedal-assist Giant LaFree Lite. I love it. I want to solarize it if possible before I bike from WI to MA in Sept. Can you send me any info you have on DIY solar panels. I want to keep the weight down. I am not looking for the biggest power, just enough to recharge the battery once a day if there is sun.

Mark said...

Hi kjcad. If you give me the capacity of your battery in Ah and volts or Wh I can help you size your solar panel(s) to meet your goal of charging the battery once per day along your route in September.

Thesolar said...
This comment has been removed by a blog administrator.
solar panel kits said...

Hi Mark. Excellent article! I just got a conversion kit for my bike and will be installing it with the help of a friend who is much better with tools than I am. The idea of using solar panels for charging is great. I hadn't thought of that.

rj said...

I am pleased to find your site. I am building a TriCruiser from plans on Atomic Zombie. I set it up to use a 49 cc gas engine behind the seat but the Government of British Columbia has made it difficult to use gas engined machines on the roads here. If I am totally stopped with gasoline power I will go to electric. Only problem is the maximum electric they will allow is less than 500W. In the overcast winters that is pretty minimal but maybe I can plug in and charge and at work since I only have to go 5 miles each way (the terrain is very hilly).

Mark said...

Hi rj. For a heavy trike with a 500W motor and very hilly terrain, my personal experience suggests budgeting 30-40Wh/mile depending on how much you actually pedal and how fast you like to go. I would also assume that you'll get about 60% of the battery's rated Ah since you'll be discharging it rapidly and to account for some extra so it doesn't wear out on you too fast. So, to make the 10 mile round trip you would want at least 500Wh capacity. That's a 48V 10Ah battery or a 36V 15Ah battery pack. You can go smaller if you don't mind the inconvenience of charging at work but I think you'll find that it's so much fun to ride that you'll want to take it everywhere, no just to work and back.

Anonymous said...

Hi Mark,

I know absolutely nothing about solar energy and motors and whatnot, but I'm in love with the concept of building and using solar-powered vehicles. Where do you suggest I start?

Thank you!


Mark said...

Start with a bike conversion kit. Search electric assist bike kit.

UkaFlesh said...

Hallo, i'm Luca from Italy. I'm trying to put some solar panel on the roof of my recumbent and on a rear trailer, monowheel.
The final idea is to put 4x60W panel, 2 on the back and 2 on the roof.
I will start to make a rear bike trailer with motor, battery and solar cells on it, so the bicycle will remain no electric when the trailer is out.

I will DIY the solar panel and i don't know how can i make lightweight and strong.

The trailer will have 250W 24V hub motors, 4x 12V 12AH battery, 24V 120W solars, with regulator. No speed control, only a switch and some anti-spark condensator.

With your experience, 120W for 250W motor give a good autonomy drive?

Thanks so much, you are a pioneer!

Mark said...

Ciao Luca,

Let's see 250W motor, 120W of solar panels... The thing to keep in mind is that the solar panels will not be driving the motor directly. First, figure out how far you want to go on a single charge and how much electric assist you want. My own experience has been from 8 Wh/mile to 25 Wh/mile depending on my average speed, hilly terrain, how much I pedal, etc. WIth a 250W motor, you're likely closer to the lower part of that range. Assuming 15Wh/mile, your proposed 4 x 12V 12Ah battery back has 576Wh... more realistically, it's going to be about 80% of that so maybe 460Wh / 15Wh/mile = 30 miles (almost 50 km). If that range sounds good to you, you probably have the right size battery.

Now let's talk about the solar panels. You could recharge your battery pack with a single 10W solar panel but it might take 2 weeks of sunny days. Keep in mind that a "120W panel" only produces 120W of output under Standard Test Conditions (STC), which are not very realistic: 25 deg C cell temperature and 1000W/sq m irradiance. Actual cell temperature is going to be about 20 deg C above ambient (dark things get hot in the sun!) and 1000W/sq m irradiance is what you get from about 11 am to 1 pm. The rest of the time, it's less. If your panel isn't directly pointing at the sun, it's much less.

As a reference, rooftop mounted solar panels on your house might produce somewhere between 60-80% of their nameplate watt rating from 9am to 3pm because of actual operating temperature, less than full irradiance and less than ideal angle to the sun. This is normal.

So, a 120W panel might take 6-8 hours to recharge your battery in full sun. If this sound reasonable to you, then a 120W solar panel is the right size for you.

My advice is to build the trailer with just batteries and motor and ride around with that for a few weeks. Charge the batteries with a charger you plug into a wall outlet. Once you get an idea of your system can do, start planning your solar upgrade!

Anonymous said...

A real solar powered e-bike running on solar energy alone, without any batteries..
Youtube video