Experiments in ultralight solar

[Ryan H.]

I’ve always been fascinated by solar. The idea of getting free and unlimited energy from the sun is just plain cool! The problem with current commercially-available solar panels is their weight. They just aren’t light enough to fit into an ultralight backpacker’s pack. To my knowledge, there just aren’t any truly light weight options out there (with any real wattage at least).

I’ve been playing around with MYOG solar panels over the past year and wanted to share some of my findings. I’ve built half a dozen panels out of different materials, using varying techniques and have learned quite a bit. I’ve honestly had mostly failures, but have finally arrived at what I think is a very promising ultralight solution.

Pretty much all solar panels are made up of the same components: solar cells, a stiff backing material, a plastic cover to protect the cells, wire and a usb power module. The heaviest component here is typically the backing material. Regular solar cells are super fragile and will shatter apart when bent/stressed. This is why very stiff (and heavy) materials must be used.

I stumbled across Sunpower solar cells after doing a bit of research last year. Sunpower cells are unique in that they run all of the bussing and power contacts on the back side of the cell. This does a few things. It allows the cell to produce more power at a physically smaller size which is cool, but more importantly, the copper backing prevents the cells from shattering apart. You can bend a Sunpower cell in half and it will still continue to produce power. Watch the first three minutes of this demo video to see what I mean.

Using Sunpower cells is the key to building an ultralight solar panel as we can now use lighter materials for the backing. I’ve tried unsuccessfully to use cardboard, balsa wood and carbon fiber sheets for backing. These materials are either not waterproof, too heavy and/or too expensive.

I’ve settled on using Depron foam which has been working great. Depron is commonly used in the hobby industry to build things such as RC airplanes, etc. It has a very tight cell structure, is waterproof, and is super light weight. It comes in a variety of thicknesses (1mm, 2mm, 3mm, 6mm).

The next heaviest component of a solar panel is typically the plastic covering that sits on top of the cells. This helps keep the cells free of dirt and moisture. Instead of using heavy plastics, glass or EVA film, I’ve settled on using heavy duty freezer wrap (Freeze-Tite). This is similar to plastic wrap but a lot tougher.

A solar panel is typically made up of multiple “modules” or groupings of solar cells. For example, in the commercial panel below, there are four modules.

The graphic below shows the weights of the various parts that make up each of my modules.

Each 5” x 5” module contains a full 3.5 watt Sunpower cell. This cell can be cut into smaller pieces depending on the size and output of the panel we're making. I don’t want to get too technical on the electrical specs right now, but the goal is to get to around 7 volts with our panel. We then use a step-down module to bring the volts down closer to 5 which falls within the USB specification. Each solar cell is rated at 0.58 volts so if we run 12 of these together in a series, we’ll end up around 7 volts total. Read more about serial and parallel wiring here if you’re not familiar with it.

So with the goal of having our solar cells cut into 12 pieces, we can assemble varying sizes of solar panels such as the configurations below. Each module weighs approximately 0.41 oz, and each solar panel is assumed to have a single USB output. Additional USB outputs can be added at a cost of 0.14 oz each.

An important note on wattage. Solar panels are typically rated by the maximum potential wattage of their cells under ideal sun conditions. In our case, each whole 5” x 5” Sunpower cell should be able to produce around 3.5 watts. Unfortunately, in real word conditions we get much less final power output coming into our USB-powered devices when all is said and done. This is due to a variety of reasons, some of which include:

• Solar cell not facing directly towards the sun. This could be due to poor panel aiming and low sun angle in the sky which is common in shoulder seasons.
• Hazy/cloudy skies will reduce the strength of the available light
• Putting anything over a raw solar cell, even something as thin as Freeze-Tite, will block/reflect part of the light. I estimate a 5% loss when using Freeze-Tite.
• The kerf of the blade when cutting cells removes power-producing material! I estimate 2% loss (per cut) when using a diamond cutting disc.
• Energy loss during the step-up or step-down process in the USB power module. This is the biggest cause of energy loss.

We have rainy skies here in Portland this week so I haven’t had much of a chance to test my latest panel, but in general, you can assume to get around 50-60% of the rated wattage under good sun conditions. So if the panel is rated for 21 watts, assume you’ll actually be getting around 10-12 watts of usable power. Fortunately this is still plenty of power to work with to charge things like mobile phones in the back country. Most wall chargers for mobile phones charge at 5 watts (1 amp).

Finished 21 watt solar panel (2.75 oz)

Two usb power modules illuminate when there’s enough light.

Sunpower solar cell cut in half. The foam is cut a little wider, a touch fatter than 5”.

Each individual module is about 3.5mm thick so all together it’s about 21mm or 13/16” thick with six modules in this solar panel.

Final weight of completed solar panel is about 2.75 oz. (78 grams). The panel weighed 2.6 oz before adding a second usb module.

The USB step-down power module can easily handle 2 amps of output (10 watts) and works with Apple products. Plastic milk carton cap was cut down and used to cover the module.

Photo below shows the depron after being cut. There are six 2mm sheets and six 1mm sheets. Foam for each module averages just under 0.1 oz each.

Radiused-corners were sanded down by hand (mainly for aesthetics). A drywall sanding brush worked really well. I used a tupperware lid as a guide. Depron sands really nicely.

Make-shift dremel table with diamond disc used to cut solar cells and depron.

Solar cells after being cut in half. Six full cells would normally weigh about 1.5oz, but with the blade kerf removing material, we’re a bit lighter now.

Cells with wire soldiered on. I used solid 26awg copper wire.

Freeze-Tite wrap being taped on to protect the front of the solar cells. I had some 3M 9485 double-sided tape laying around from some past cuben projects and it worked great (this is the tape Zpacks sells). My tape was 1/2” wide so I cut it in half for 1/4” strips. The wrapping process is a little tricky. You want to get the wrap tight but not pull too hard as to crack the solar cells. I had a hard time getting the wrap totally taught which gave me a few small wrinkles. This is purely an aesthetics thing though and doesn't affect operation.

Modules after being wrapped with Freeze-Tite. The wrap got a little poofy around the radiused-corners so I used some cuben tape to pull it back.

A strip of cuben fiber is used as a hinge that holds the two modules together. The yellow wire is super flexible stranded wire that works well for the constant opening and closing of the panel. Regular solid/stranded wire is brittle and will break with only a few folds back and forth. This is 26awg wire that has 64 strands of 44 gauge wire. Running the wire in an “S” configuration helps mitigate the bending force on the wire.

Cutting out holes for the two USB modules.

Anyway, I think that’s about it. I hope this info is useful to someone. If there’s interest, I can put together a materials list. Let me know.

Help needed from the community:

• Ideas for stronger/lighter materials
• More efficient voltage/USB module combinations
• Cheaper alternatives to any materials
• Cleaner ways to cut the cells (laser cut?)

Thanks!
Ryan

–[UPDATE]–

Materials List

Sunpower solar cells:
I’ve purchased most of my cells from TG Solar off of eBay. He ships priority mail out of Ohio in the US which means you can get the product in just a few days. He only sells ten packs though. There are other Sunpower cells for sell on ebay from China or elsewhere that will sell individual cells for a little more per unit. Definitely buy one or two extra though in case you mess one up while cutting.

Cost: $4.50 per cell
Sources:
TG Solar on eBay
Other eBay

Depron foam:
I’ve purchase all of my depron online through rcfoam.com. RC Foam is awesome except for their shipping is crazy expensive. They have to carefully wrap the product and ship it in large boxes so shipping ends up being like $13+. It’s hard to get the depron to your door step for less than $20. I think there are a few suppliers that ship out of the UK that might be cheaper if you live in that part of the world. I have not used them, but eBay also has some pre-cut depron available. This might be a cheaper route to go depending on how much you need. 1mm depron is more expensive than 2mm. If you want to save money and have a stronger panel, you can just buy 2mm depron and use it for both depron layers. I mess up half the time when cutting depron. Buy more than you need.

Cost: $15-25
Sources:
RC Foam
eBay

USB power module:
This is hands-down the best module I’ve found and it happens to be the smallest and lightest too. I’ve only found these shipped from China. Usually takes 7-10 days to reach me in Portland, OR.

Cost: $3.12
Sources:
eBay

Freeze-Tite:
I have not experimented with other heavy duty plastic wraps, but this could easily be substituted if you can find something cheaper.

Cost: $10.74
Sources:
Amazon

26awg solid wire:
I typically use solid copper wire for soldering onto the solar cells. Stranded wire if fine too if you have some laying around. If you’re doing a small panel, you can probably get away with even 28awg.

Cost: $2.73
Sources:
eBay

26awg super flexible stranded wire:
This is the yellow wire I used to connect each module. It’s CRITICAL that you use super flexible wire for this application. Regular wire is not designed to be bent back and forth thousands of times and will eventually break leaving your panel dead in the water. The only source for this wire that I have found is this random dude online. I was a little hesitant, but paypal’d him $9 and he shipped it out next day. $9 is a lot for what it is, but I don’t currently have a cheaper source. Please share if you have a better place to buy wire. Just make sure it’s similar with 64 strands of 44 gauge wire.

Cost: $9
Sources:
Random dude

Double stick tape:
3M 9485 double stick tape. This tape is really expensive, so if you don’t already have some laying around, you’re probably better off finding a cheaper alternative.

Cost: $28.95
Sources:
ZPacks

Cuben fiber:
0.51oz or 0.74oz cuben used for the hinge between each module and also for cuben tape. Again, this is really expensive and some other material could be substituted if you don’t already have some on hand.

Cost: $29.95
Sources:
ZPacks

Foam Tac glue:
I used this glue to attach the solar cells to the depron. It goes on wet and has a minute or two of working time which allows you to slide the cell around and get it centered nicely. I haven’t tried it, but regular silicone from Home Depot would probably work just as well and be much cheaper.

Cost: $3.95-$12.95
Sources:
eBay
RC Foam
Home Depot

Spray adhesive:
I used 3M Super 77 for gluing on the final depron panel. I think the Scotch brand is probably the same thing and is cheaper for a small can or maybe you can find cheaper locally. Other general spray adhesives may work fine too. Do some testing on scrap pieces to make sure the adhesive doesn’t eat the foam. 3M Super 90 will not work!

Cost: $6.88
Sources:
Amazon

Voltage Tester:
To accurately see how much power your panel is producing, you’ll need something to test it with.

Cost: $6.95
Sources:
eBay

Misc. items needed:
Hole punch
Solder/flux
Soldering iron
Disposable rubber gloves
Multi-meter
Milk jug cap or other plastic bottle cap
Exacto/utility knife
Dremel/rotary tool with diamond disc (check Amazon for discs)
Sand paper

[Jerry Adams]

what's the USB module?

do you buy it or does it come with solar panels?

[Franco Darioli]

Ryan,
Let me state the obvious : there will be some people here interested in buying such panel.
I am one of those…
The only comment I can offer is :wow !
(BTW, I have friends that are about to buy a solar system for their new house and have just found out those panels are available here in Australia. Thanks for the tip)

[Andy E]

+1 on buying. Keep up the good work! this is exciting.

[Bob Gross]

"what's the USB module?"

There are some standard (cheap) USB modules available online. They take around 5.5-6.0 volts in and produce a standard 5v USB output.

One vendor is Brown Dog Gadgets.

–B.G.–

[Mike In Socal]

Ryan,
Nice work on this. I'm definitely interested in a parts list. I spent some time flying RC gliders, and understand the need for stronger/lighter materials. You'll find that adding strength will add weight but you can keep it to a minimum. For starters, if you wanted to strengthen the foam backing, you could add a layer of 3/4 oz fiberglass cloth with some finishing epoxy. Another option would be to just fiberglass the edges. It all depends on how durable you want to make the final product and the materials you select. If the panels are just sitting on a rock in the sun, then the design may be fine as is. If you want to hang it off a pack while hiking then more strength would be needed.

[Richard Nisley]

Ryan,

Excellent work; thank you for sharing. You solicited help from the community relative to more efficient voltage USB combinations. The lightest solar charger weight is secondary to actually charging contemporary devices at their potential rate if it is less than or equal to what the solar cell is putting out.

All the USB connectors are the same, with 4 pins. One pin is Ground, two pins are Data (D+ and D-) and the last pin is 5V power. The Ground and 5V pins are used to provide power to whatever is plugged in. The two data lines are used to transfer information back and forth. At least for Apple, the two data lines via a fixed voltage on each data line, tell the Apple charge controller how much current to accept. Using a 2.0V & 2.0V setup results in a 500mA charge rate and 2.8V & 2.0V setup results in a 1 Amp charge rate.

Some solar charger's USB controller can't provide the signals to a device's Apple charge controller to step up the utilization to a maximum of 2.1 amp or 2.4 amps as per Apple's standard wall chargers.

Does your USB controller specification say it is capable of enabling Apple's devices to accept the current that your solar cell is providing? Have you tested this capability to see if it is true? What USB controller are you using?

[Bob Gross]

Richard, I am no Apple expert. However, I think that there is a gadget that you can put on a non-Apple charger (or solar charger) USB output that will fool the Apple USB input to accept what you want.

–B.G.–

[Cameron M]

Great work, thanks for sharing! These Sunpower cells appear to have a lot of potential.

I think the form factor is cute but ultimately unwieldy and not practical for charging while walking, and additionally each hinge adds unnecessary electronic fragility as well as too many seams for water invasion. A single rectangle 10" x 10" or 10" x 15" might be more practical for hanging off a pack, would greatly lessen the stress on the wiring, and could allow a single PVC protective sheet on the front and back all sealed with one perimeter. Or perhaps a 10" x 20" with a only single fold, where the panels fold to protect their faces when not in use.

[Jerry Adams]

I have noticed that some devices pull as much current as possible until the voltage goes down to 4.8 volts (or so).

Like, I have a USB power supply that charges like this. After it gets charged, it will charge a higher current device.

This would work good with a solar panel. If it gets shady, it'll still deliver a little current. Or, set it up at base camp. Over the course of the day, hopefully, your USB power supply will get charged. At the end of the day, when the sun has gone down, you can use it to charge your iphone which you carried with you during the day.

Don't some devices, like iphones, try to get a large current, and if the USB charger can't deliver it just shuts off?

[Gary Yelland]

For me solar tech is just not there yet for me.

I love the work you have done, and at 78g it is really light weight.
I not so keen on the bulk and size.

Obviously you need a lot of surface area to get a decent output,
In the uk with a lot of cloud it just doesnt really work the outputs are just too small unless your panels are huge.

Most new devices are using 2A charge as standard.
But its really important that your source is also current limiting, as sometimes the devices will sink a lot of current and not regulate it very well ending up with too hot battery or worse.

The robustness will be a key factor and that will need a lot of testing, lots of flexing joints etc.

I was thinking that sticky back clear plastic you cover books with, its very strong and bonds well, but a one time use i think as it wouldnt come off.

For me when technology gets to the size of one panel and puts out 2A on a cloudy day then i think its interesting. I think the nm photon antennas is interesting tech for this or even quantum antennas for efficency. Or massive like woven onto the sureface of fabrics.

Until then i think i will use a power brick when i need it.
Will be very interested to see how this progresses. Looking forward to you custom 3D printed cases lol :)

:)

[Cameron M]

Aside from heavy and inefficient cells, the bugaboo with DYI setups has been the controller with direct-to-iPhone charging. I have been using this simple setup for the last two years: One 6 volt 830 milliamp cell with a diode and a brown dog sourced USB charge controller. 5.6 oz. Some devices can take very sloppy and variable charges, like a cheap iPhone 4s battery charger I had, or the Anker battery I am currently using. So you can plug them in all day and they will charge despite the variable current. While the iPhone 4s itself was too picky and would not charge directly, I was pleased to discover that the iPhone 6 seems to be more tolerant and with good sun this setup works, but as yet I have no trail experience with it.

When you hike, the delivered power is generally all over the place. With the right controller and a larger setup delivering more power, the better the chance that enough power will be delivered continuously to satisfy the iPhone. What may be called "inefficient" in good sun may also be a requirement for continuous charging in variable situations. So for me there are three levels of demand in considering how large and heavy a system to design): (1) deliver just enough usable USB power to meet my power needs to a battery or device like the Steripen; (2) deliver enough correct power for direct-to iPhone charging when in acceptable sun; (3) deliver enough power for an iPhone with wildly varying charges while walking. I have only achieved (1), and at 5.6 oz the benefits over a battery are still specious.

These new Sunpower cells may satisfy the problem of cell weight. I don't think the housing has to be much of a design or weight issue. The controller is the most difficult challenge for those of us who can't make this stuff. I have been meaning to check out this controller, it looks promising:

http://www.seeedstudio.com/depot/LiPo-Rider-Pro-p-992.html

Pics of my current setup, soon to be replaced I hope by Sunpower cells!

[Gordon Gray]

Totally cool, Ryan.

I would also like to buy one.

[Ryan H.]

Thanks for the feedback, guys. I'll update the original post with a material list later today.

@Mike:
Funny, I was just looking at some fiberglass cloth last week. The cloth itself is quite light. I've seen it as light as 0.5 oz/yd. The problem is the epoxy. I have not tested it, but would think it would add a bit of weight. I guess maybe if you did a very light coating of the epoxy and kept it sandwiched in between the two layers of depron, it could work. Maybe worth a try!

@Richard:
You're absolutely right about the data pins for Apple devices. The correct amount of resistance has to be applied to those two pins for Apple devices to start charging in their fast mode. I'm an Apple guy, so I purposely only seek out USB power modules that work with my devices.

Here's the current USB module I'm using which has been working great to charge my iPhone and iPad. As long as you're using an iOS-compatible USB module, an iPhone will draw as much power as it can, up to its max charging rate. The iPhone 6 comes with a 5w wall charger, but the phone is actually capable of charing up to 7.5 watts (1.5A). Try plugging your iPhone into a higher wattage charger such as a newer Macbook Pro or an iPad wall charger. If you use a voltage tester, you can see the difference:

@B.G.
I bought the Brown Dog Gadget's USB module last year and was pretty disappointed with it. It's physically kind of bulky, but worse, it only charges up to half an amp (2.5 watts).

I think this is the device you're talking about. I picked this up a while back but have never found the need to use it.

@Cameron
Agreed about the form factor not being ideal for charging while walking. The "alt" version of the 21 watt panel would be better since it's more compact. In general though, I don't think anyone would want to use a 21 watt panel for charging while walking. At 21 watts, my panel or a commercial equivalent are frankly just physically too big IMO. I have built some larger form factor panels like you're describing and have ultimately steered away from them. You're right about less hinges/wires on a larger module, but the major drawback is rigidity. When using ultralight materials like this, large panels will tend to bend (and break) much easier. The cool thing about the small 5"x5" form factor, is that stacking numerous modules together when closed/stored really adds a lot of strength to the system. It's makes storage a heck of a lot easier too; You can tuck a 5"x5" panel in a side pack pocket or a back mesh pocket easily and without worry. Make a small cuben pouch for added rain protection, and you're good to go. I'd argue your large form factor panel would be more susceptible both breakage and rain as you can't properly store it in a down pore or when not in use.

If I were to do a larger size, I probably wouldn't go bigger than a 5"x8" panel like this one I built last year:

@Jerry
I think you're right about voltage needing to be high enough or charging shuts off. The iPhone is super finicky when charging on variable voltage. I'm not sure what the actual threshold is, but charging seems to shut off when voltage drops somewhere in the low 4s. I previously built a bunch of 8 cell panels (4.64v) and used a "step-up" module oppose to a "step-down." Besides the USB modules being less efficient, with the voltage already on the lower side, any decrease in light or even walking in front of the panel and temporarily shading it, would cause the iPhone to exit its fast charge mode. Starting with a higher voltage, as in my current design, seems to help a lot. There's basically more voltage to be had so the chance of the voltage dropping into the low 4s is less likely. To answer your question though, the iPhone can absolutely charge on a very low current (and even zero current), as long as the voltage stays up.

My personal charging plan of attack is to carry a large wattage panel, like the 21 watt version, and only pull it out when stopping for water or lunch breaks. The 21 watt panel, even with less than perfect light conditions can produce 4-5 watts of power which is the same as a wall charger. If you have excellent sun, you can actually charge your iPhone faster than if you were at home (up to 7.5 watts). So I basically try to hit it hard and fast and be done.

I'm still on the fence about the "charge while walking" method. I need to do more testing, but I definitely wouldn't attempt to charge a iOS device directly while walking. An external battery pack works really well for this since it can trickle charge and not complain about varying voltage/current. I recommend this battery pack. 3000 mah will just about charge the iPhone 6 one time. It weighs 2.6 oz and as a bonus has a built in flashlight that's very bright. If you use this as your primary light, the battery portion is almost free weight.

[Ryan H.]

A few more thoughts on charging while walking…

If you need to attach the solar panel to your pack, you obviously need something to tie onto. Before you glue on the back panel, add some cuben fiber loops sticking out.

One bummer about running the solar cells in a series is that shading just a single cell brings down the power of the entire array. For example, if you take your hand and try to shade just one of the cells, it's the same as if you're shading all of them. This is just how the electric principles work when things are wired in a series.

This is problematic for charging while walking because any shadow cast anywhere on the panel, like from your head, will greatly reduce power output of the entire panel.

I have not tried this but it seems like if you're set on a charge while hiking approach, having multiple panels tied together in parallel could work nicely:

So basically each module on the right and left would run 12 cells in a series and then be connected together in parallel. If one of the modules is shaded, the other will not be affected. And if you're not worried about looking like a total geek, run this setup in a upside down V shape and use it as a hat : )

[Cameron M]

"One bummer about running the solar cells in a series is that shading just a single cell brings down the power of the entire array."

It is also true that one broken wire brings down the entire array. It happened to me! So I would steer towards a smaller number of fragile connections, and be conservative about packaging.

The charging while walking demand is difficult or nearly impossible with the iPhone, but as you say works fine with trickle charging other devices. The problem is that while you can think of the batteries in the Bosavi light and Steripen as "battery reservoirs" that can get you through a week of no sun, the iPhone battery may only get you 2-5 days, depending. So then you have to add an external battery, and then you've added weight, and another device.

At any rate, I think these cells are a promising direction and look forward to hearing about your progress.

[Stephen Parks]

Ryan, nice work.

I have no experience with solar cells, but for for laser cutting service you may inquire with a-laser. They specialize in laser cutting of thin materials, will cut materials that you send them, and, conveniently for you, are on the west coast. I haven't used them myself in several years, but they were quick and surprisingly cheap then. You would need to discuss special handling and packaging procedures with them.

I can only guess that the cells can be laser cut, but it seems likely. I cannot guess whether there may be some damage to the cell from the laser. I expect they can also cut your foam pieces if they aren't too thick.

http://a-laser.com/cutting-services/

[Ryan H.]

Thanks for the link, Stephen! I'll check that out.

I've had some sunny days here in Portland this week and had a little more time to do some testing. Unfortunately, my latest panel is not producing the power I had expected. I think I've pinpointed this issue and it's related to the small wire gauge I'm using. I'm guessing there is too much resistance with the thin wire. I'm not sure what the ideal wire gauge is right now, but I'd recommend going thicker if you attempt one of these panel. I'll report back when I'm able to confirm a better gauge.

[Richard Nisley]

Ryan,

What are your current design's volts, amps, and total wiring distance?

See http://www.freesunpower.com/wire_calc.php

[Ryan H.]

The volts once the panel is warmed up hover around 7.1.

Amps in theory could be as much as 2.8.

I'm not sure of the exact feet I have in wire, but I would estimate around 6 when I add up all of the short sections.

Cool tool, thanks for posting that. It only goes down to 12 volts, but given my above numbers, it says I should use 21 gauge wire. If you consider 7.1v, the gauge would need to be even heavier. Maybe 20awg would be sufficient.

[Bob Gross]

The part about 3-5% acceptable loss is the tricky part. It kind of depends on the load or the voltage regulator that is following the solar panel. Just the same, loss is loss, and that is generally undesired.

I used to have a job where I had to calculate the optimum wire gauge to cover a length with a certain amperage, and the result was in terms of a small allowable voltage drop, like 0.12V from 48V. If I used a small wire over a long distance, I ended up with too much voltage drop, and that led to other problems. There is also a small temperature factor.

As an experiment, measure your voltage output at present, then double up the wires. Each doubling of the wires is equivalent to going to the next bigger even wire gauge. If you see only a tiny change, then you can ignore the wires. If you see a significant change, then it might be worthwhile to change the wires.

If all else fails, you can change out the copper wires for silver ones.

–B.G.–

[Bradley Danyluk]

Interesting stuff. Very light.
I've been playing around with some panels and have some ideas to consider.

Your design has two apparent "flaws," which may or may not be bothersome depending on what you want:
1) It's quite bulky when packed.
2) It is quite likely useless in less-than-good weather or shade.

I'd like to propose a design, which I may end up building, that would solve both those issues, and keep the low weight, but trade them for the following issues:
1) High surface area per watt (thus difficult to get exposure on a backpack for 10+ watts).
2) Much higher cost.

Check out the PowerFilm OEM panels:
http://www.powerfilmsolar.com/products/oem-comparison-chart/
In particular, the RC Aircraft modules which have no backing. They are 0.2oz for a .72w panel.
I've been playing around with these panels, and they need no backing at all. They are paper-thin and designed to be flexible. You can bend them or roll them as tight as you please, and they won't break. So your packing bulk issue, and the weight of the backing, is gone.

They are amorphous silicon panels. These panels have all internal cells wired internally in parallel rather than series, which means they are way less affected by shade or even overcast conditions. I have tested this – they still provide *useful* output in poorer conditions, even indoors, you just need more of them to make up the power differential.

Another reason for this is their voltage is somewhat higher. It reaches over 10v in direct sun. Why does this matter? Because if you're using a buck converter to get a USB output, it stops working when the input voltage drops to around 5v. And when the sun exposure drops, voltage drops. With lower voltage panels, it won't matter how many you have wired in parallel, once the voltage drops, you get zero output from the USB. But here, even in poor conditions where the panel is only putting out 55% of its normal voltage, the buck converter is still able to work and provides a 5v USB output at a lower amperage. Again, the amperage can be increased as desired simply by adding more panels in shady conditions.

The real downside is they are bloody expensive. Like $40+ per watt expensive. I think my end goal is to get around 25 watts worth of them, which could keep a Windows tablet or maybe even an ultrabook going indefinitely in the field, through whatever conditions. I have unique goals (content creation on the trail) that not many will share, so this is worth it to me… eventually. The weight per watt will be comparable to your setup here (slightly heavier), and folds/rolls very small with just the wiring and cuben tape holding the panels together.

[Bob Gross]

"bloody expensive. Like $40+ per watt expensive."

Good Grief!

For over 15 years now, my cost goal for any sort of solar panel has been no more than $10 per watt, and ideally $5 per watt. Some of the latest panels are cheaper than $3 per watt.

A lot of cost at the solar panel end could be saved by reducing output load.

–B.G.–

[Cameron M]

The Powerfilm appears to be what Bushnell uses, and I can't find anyone using it that manages to directly output 5V USB. Every application I have found using Powerfilm requires an intermediate battery, which begins to mitigate the weight advantage. The cells also seem to require a lot of area for watts produced, perhaps too much to be practical on the trail. If you manage to get a 5 watt unit working that weighs less than 8 oz and outputs USB usable power, let us know!

[Bradley Danyluk]

I've been going direct to USB from Powerfilm panels for years now. It works as well as any other panel; there's nothing special about Powerfilm panels that somehow make them unsuitable for it. A watt is a watt is a watt.
Yes, they need significantly more surface area than mono or poly cells. But they're also far lighter per watt than most.
I tore apart my last module based on Powerfilms to run some tests on the panels and potentially use them in a new project, so I don't have any photos right now. But I was getting 10 watts out of a 7oz module (on heavy nylon backing).

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