Compact 20W Folding Solar Panel (min. 20W)

Given a choice, you ALWAYS want to charge your device off a Portable USB Power Bank. Use solar panels to trickle charge your device only as a last resort.

Power Banks provide the fastest and most consistent charge to your device, because of their built-in charge monitoring circuitry. Re-charge the Power Bank with a solar panel only if you don’t have ready access to a wall outlet or vehicle cigarette lighter port. If the Power Bank is completely discharged, then you could use the solar panel directly to trickle charge your device, but it is best to have the solar panel trickle charge the Power Bank first, instead of the device directly. The primary reasoning is that most (99%) portable folding solar panels do not have a voltage regulatory built-in, which means that the charge varies, relative to the sunlight. This variance, over time, puts a stress on the battery chemistry.

The combination of a solar panel directly attached to a power bank is a great idea in theory, but if either the power bank or solar panel fail, then you basically need to discard the entire unit. So, it is better to keep the two components separate and modular. Also, a power bank is much more compact to move and carry around, without the attached solar panels (the exception to the rule is the Powerfilm LightSaver Max Portable Solar Charger, which costs around $380USD and is difficult to find).

Based on my testing of a variety of crystalline style portable folding solar panels from Amazon, the Anker has a very good quality 21W portable solar charger which has a better-than-average voltage regulator built-in. It’s this small circuit that separates many of the made-in-China fold-able solar panels found on Amazon, from the USA designed versions. More on this below.

PowerFilm‘s thin film F15-1800 30w is the solar panel I’ve settled on for my solar charging needs. PowerFilm panels use amorphous silicon (a-Si) thin-film solar modules (like on a solar powered calculator), versus crystalline solar cells. The efficiency of amorphous solar panels is not as high as those made from crystalline solar cells, so the amorphous panels are larger, but cost quite a bit more. The advantages of amorphous panels in a bug out situation though, far out weight the higher efficiency of the smaller crystalline panels. The advantages include:

• they charge better in overcast conditions — which is a more realistic scenario than expecting full sunlight each and every day
• they are bullet proof (literally!); Powerfilm panels stand up to abuse and will continue to produce a charge, unlike crystalline panels, which when damaged, pretty much make the entire folding solar panel set, toast
• they are flexible; they can be rolled up or folded up into a much thinner profile
• because amorphous panels are thin, they are much lighter to carry
• Powerfilm’s version of the solar cells are designed to output 12V; this allows for charging vehicle batteries; in addition to using cigarette lighter accessories such as a 5V 2.A USB charger to charge your mobile phone

In terms of reasonable performance, you want at least a 20W panel to charge a mobile phone in a reasonable amount of time (4-6 hours). 30W or 60W folding solar panels are preferable, but much more expensive and larger when folded.

To calculate the actual amperage (which is what determines how quickly something can be recharged), divide the rated watts of the panel by the voltage. For example, a 20W panel will theoretically produced 4A at peak performance given that USB devices operate at 5V (20W / 5V = 4A). Based on this theoretical value, I usually take 60% of it to get an ACTUAL operating value you would get in the field (4A x 60% = 2.4A real-world expected charging capabilities). Then, if the panel offers up 2 or more USB connections, I divide the real-world result by that many ports (solar panel with 2 USB plug-in ports = 2.4A / 2 = 1.2A typical output).

The listed wattage rating for a folding solar panel, is the optimal output during ideal sunny conditions between 11am-4pm, with the panel perfectly angled towards the sun. Unfortunately, you will rarely get this kind of output on a consistent basis. In theory, when trickle charging a device directly off a solar panel, and some clouds roll in and block the sun, the solar panel’s voltage regulator is supposed to compensate and properly lower (step-down) the output amperage while continuing to trickle charge the device. In reality, when clouds appear, the less expensive folding solar panels from China, simply shut down the solar panel output, producing NO charge. This means you have to be very conscience of any clouds, and need to continually monitor if the solar panel is still working or not, which sort of defeats the purpose of having a solar panel automatically and continuously charge a device back to full capacity. If this happens to you (clouds appear), then you will need to manually unplug the device from the solar panel, wait a few seconds, then re-plug it back in, in order to restart/reset the trickle charge coming out of the solar panel. You’ll assume that because the sun is back out, that the solar panel is automatically going to start charging your device again, but this will most likely not be the case.

After much testing of several Amazon folding solar panels, it appears Anker’s solar panel circuitry is one of the few that adjusts for cloudy conditions and continues to produce output through a wide range of sunlight conditions, whereas the lesser brand solar panels do not. One last important note: If you are going to trickle charge a device or re-charge a Power Bank with the solar panel, leave the solar panel exposed to the sun for at least 1-2 minutes first, BEFORE you plug anything in. This allows the capacitors in the voltage regulator circuitry to “charge up” or create/fill-up a buffer so that the trickle charge is more consistent. Otherwise, plugging in the device or Power Bank immediately after setting up the solar panel, will not give you the best trickle charge, because the device will be immediately draining these capacitors and not letting them establish their full capacitance.

With all that technical detail behind us, the bottom line is that typically, a 20W solar panel will produce about 1.2A to 1.8A consistently at around 5V while the sun is out with no clouds. Compare that to the quick charge wall warts that operate at 2.4A or even 3.0A now-a-days.

A very useful tool to pair with a solar panel is a USB Multimeter, which measures the voltage and amperage being sent to your device. I definitely pack one of these at all times with my solar panel kit to help troubleshoot, test, and estimate how long it will take me to charge my mobile phone and other USB chargeable devices.