Category Archives: Product Evaluations

Charging directly from a Solar Panel

Can I use a Solar panel to charge my phone or tablet?

There is a lot of interest in charging tablets and phones directly from a solar panel, without the expense of an external battery and charge controller. We’ve done some testing on a variety of phones and several tablets that charge over a micro-USB connector at 5 V to determine what is practical.
We tested using one of our 12-V, 15-W Solartec panels and two different USB car chargers, one from Aukey, and the other from Anker. The car chargers are essentially dc-dc buck converters, and both operated similarly. We chose models that were rated for 12 V to 24 V, so the open circuit panel voltage of 21 V wouldn’t damage them.Anker-USB-Auto

The Bottom Line

A 12-V panel with dc-dc converter doesn’t work well for most devices, and this setup is not recommended. However, most phones, and some tablet devices will charge nicely from a solar panel rated 5 V, and 5 W or more.  Some tablets will need a panel rated over 10 W or they won’t charge. The characteristics of a “12-V” panel used with a dc-dc converter cause problems. (See below.)

The Technical Details

We can’t make any universal statements about the useability of direct solar charging because it all depends on the design details of the power circuits inside your phone or tablet, and manufacturers don’t publish the technical details you need to know. Generally speaking, many phones will charge nicley from a solar panel, and a minority of tablets will also.

A problem arises when the amount of power available from the panel is less than the device requires. Depending on how the device has been engineered, there are three possibilities:

  1. The device will continue charging at slightly reduced voltage, using whatever current is available.
  2. The device will stop charging, and not retry until power is completely disconnected and reconnected.
  3. The device will repeatedly attempt to charge and quit in a several-second-long cycle.

A lot of phones do #1 or #3, which works fine if they have their own dedicated panel. If a cloud passes over, they will continue charging when the sun comes back out. Many tablets and most Apple devices seem to do #2, which makes charging directly from a solar panel unworkable. An electronic device which cuts the power for a second about once every minute might make such devices charge reliably, but I know of no inexpensive devices which would perform this function.

The problem with using a dc-dc converter to produce 5 V from a 12-V solar panel

Direct Charging ProblemWe recommend you do not try and use a 12-V panel to charge 5-V phones or tablets. Here’s why:

A “12-V”, 15-W solar panel puts out about 13 W under typical conditions, but this is at the max power point of 15 V or so. Below the max power point voltage, the panel acts like a constant current source, so the power curve drops linearly as the voltage drops. The power curve looks like the green line, with a big hump around 15 V. Under full sun, a dc-dc converter can easily provide 2 amps at 5 V, (10 W) and the panel is operating to the right of the max power point. This works fine until a cloud passes over. Now the panel’s power curve looks like the dark green line; it can’t produce the 10 W the device wants. The device keeps charging at whatever amperage the panel can supply (~0.1 A) which pulls the panel voltage all the way down to 4.9 V. At 4.9 V, the panel is now operating at a voltage way below it’s max power point, and even when full sunlight returns, at ~5 V the max amperage the panel can create is about 0.8 A; the device is taking all the power the panel can produce and the panel voltage never gets a chance to climb back over 15 V. The device will continue charging at 5 V, 0.8 A = 4 watts, instead of 10 W. The panel needs to be disconnected briefly and reconnected in order for the voltage to return to the high side of the power hump. A solar panel with it’s max power point at 5 V will not have this problem. I would recommend something like the X-Dragon 5V14W panels for charging tablets.


Lenovo Yoga Tablet 2 Evaluation


GTIS power systems has been looking for a replacement we can recommend for the low-powered Lenovo X140e netbooks which are no longer available.

We bought a Lenovo “Yoga Tablet 2 10 with Windows” tablet for evaluation. It comes with a 1920 x 1200 resolution, 10.1-inch diagonal screen, and it runs on about 4 watts average, meaning it could be very cheaply powered with one of our Half-Pint systems and 30-W to 45-W worth of solar panels.

Evaluation summary

This tablet is useable for translation work in a village setting, but is not as handy to move around with as the X140e, since you need to attach an external USB hub, keyboard and mouse to get a stable typing setup.



  • Beautiful bright clear screen
  • Solid build quality
  • Under 4 W average power consumption
  • Fabulous runtime — over 12 hours without recharging
  • Good CPU speed
  • Runs your favorite MS Windows programs on Windows 8.1 or free upgrade to Windows 10
  • Probably can run Ubuntu with a little work. (Not attempted, see this thread.)


  • There are charging issues. The USB charger that comes with this tablet puts out a non-standard 5.3 volts. Chargers that put out less than 5.0 V will not work, and you must use the included cable or one with 24 AWG or bigger wires. It pulls 1.85 amps when charging. If the voltage sags below 5.0 V it won’t charge and Windows will report “plugged in not charging” in the power applet. (The Vanson 12V DC-DC converter we sell works OK.)
  • Single micro-USB port can be used either with USB charger or with an USB OTG cable to attach flash drives, USB keyboard and mouse but not both at once.
  • Included Bluetooth keyboard isn’t very good and looses connection with the tablet; aftermarket Bluetooth keyboard also sometimes had connectivity issues and pauses.
    Needs a good USB external keyboard and mouse to do serious work. I like my full sized Logitec Wireless Combo MK270, which uses a single USB receiver to connect to both the mouse and keyboard. But this means you can’t charge while you’re working.
  • No Ethernet. You need to connect over wireless, or else install a USB Ethernet adapter using a USB hub and OTG cable.


If you hook up an external keyboard and mouse via an OTG cable with USB hub, this system can be used very nicely as a micro-sized desktop replacement, but it looses it’s handy portability. I especially liked the solid feel of the system and the excellent high resolution screen, even though I need my strong reading glasses to see it well. I’m not a MS Windows fan, but Windows 10 seems to run very nicely on this system, and it was no problem to run Paratext, BART and several other programs at once on this system.

I can’t really recommend this as a direct replacement for the Lenovo X140e, but folks wanting to run on a small inexpensive solar setup could find this very workable.

Note 1: This model is being discontinued and is not available from the Lenovo site, but has them on clearance for $244. Search for SKU 8732054

Note 2: Lenovo makes a 13-inch diagonal version of this tablet which is more expensive, even higher resolution, but only uses a little bit more power. $450 on Lenovo’s website, but probably will be gone soon.

Note 3: Dell makes some 10-inch Windows tablets that could be used similarly. The Dell “New Venue 10 Pro 5000 Series” has a model with 1920 x 1200 resolution for $379.