Category Archives: Solar Panels

Portable Solar Power System Usage Scenarios

How you plan to use your solar power system affects what you need to buy, especially when it comes to the solar panels. Picture exactly how you’re going to use your system before you buy.

Most small commercially made solar power systems are designed for temporary use such as backpacking, “RV-ing,” or camping. A good example is the Goal-Zero Sherpa 100 power pack. In this scenario:

  1. The power bank is used for a few days or weeks at a time, and then sits in storage until the next trip.
  2. The solar panels are laid outdoors on the ground right next to the power bank, possibly inside a tent or vehicle for protection. The power cord on the solar panel is only a few feet long.
  3. The solar panels are brought indoors at night or when it rains, and don’t need to be durable enough to survive years of harsh exposure.
  4. Light weight is worth paying a premium for.
  5. The power bank is brought indoors in the evening and used to recharge the battery in a phone, camera or tablet.
  6. The device is not field repairable, but disposable after a couple years.

If this is the sort of usage scenario you anticipate, A Sherpa 100 or Voltaic V72 battery system will serve you well. A folding canvas-covered solar panel like our Bioenno 28 W or similar products sold on will provide lightweight power that isn’t too expensive, or you can get the deluxe Global Solar folding 30 W panel which will last a lot longer and is even lighter and more rugged. Our Villager and Half-Pint power systems can also be used like this if you pair them with a portable panel and short cord.

In contrast, some field workers have a need for a more permanent power system that can be easily transported to a very remote location where it is permanently installed. Their equipment choice is driven by different trade-offs.

  1. The equipment is heavily cycled on a continuous basis, e.g. they use it every day for years at a time.
  2. The solar panels are permanently mounted outdoors and must survive the constant exposure to the weather.
  3. A long, heavy, UV-resistant power cable must be used to connect solar panels outdoors to the power bank which is located indoors.
  4. Possibly the user’s equipment is hooked up to the power bank and charging during the day while the power bank is also receiving power from the solar panels. (The power bank is discharging at the same time as it is charging.)
  5. Though weight is still important, low cost and ruggedness are more important.
  6. Long term survivability, extended battery life and field replaceable components mean lower cost for the users.

Our Villager and Half-Pint PV power systems are designed for this scenario with LiFePO4 batteries that are rated for 2000+ recharge cycles vs. hundreds of recharges for “normal” lithium batteries. The batteries and other components can be replaced by a field technician for much lower long-term costs. We offer a variety of solar panel options, including mounting kits to fit your budget and usage scenario. We can help you choose and make sure you have all the parts you need if you email us.

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.


Villager-III – What panels do I need?

The Villager-III system is sold without solar panels, so that you can choose the type that fits your needs best. Your choice will depend on how you want to trade off cost vs. weight vs. ruggedness vs. physical size. If you’re using the Villager as a portable system you won’t need a long heavy power cable or mounting brackets. However, for this discussion we’ll consider you want to permanently mount solar panels on your roof.

The Villager III Power Bank can hold just over 200 Wh of power when fully charged; it’s designed for people who need 100 Wh to 150 Wh per day to run their computer etc. In most tropical locations a solar panel rated 50 W will generate about 200 Wh on an “average” day. But you need to be able to both recharge your battery and run you equipment after a cloudy day, and not every day is “average.” We recommend you use a panel rated 60 W to 80 W for most situations.

First, consideration should be given to the Semi-Flexible 100W Panel kit we offer. This panel is very lightweight and rugged, but is rather large, and might be awkward to transport. It will provide plenty of power, even on not-so-sunny days. The “kit” includes a wiring harness.

We also have an inexpensive kit made of five 15-W panels you can use with a Villager III, and we still have a few Unisolar roll-up panels rated 68 W. Also, there are a number of other commercial options available.  Here are some possibilities you can purchase directly from the provided links, or GTIS Power Systems can purchase them for you.

If you’re not getting one of our “kits” you will also need to purchase a cable to run up to the roof with the right connectors to connect to your panels and the Villager; an extra expense not in the quoted prices. We can create a custom cable to meet you needs, just email us.

Comparison Matrix

(See below for a description of the various panels in the matrix.)

Panel Weight Fits in Suitcase? Durability Cost
5 each 15 W Solartec 17lbs Yes -medium Fair <$100
2 each 40 W Mono 15lbs Yes – large Fair $150
Single 100 W Semi-Flex 4lbs no Very Good $216
2 each 30 W Semi-Flex 4lbs Yes – large Very Good $240
Global Solar Folding 62 W 3lbs Yes – small Excellent >$700

Thin & Lightweight Semi-Flexible Panels

100 W Semi-Flexibleflex100

This panel is especially small for a 100 W panel because it uses high efficiency cells. It’s lightweight and semi-flexible, but won’t roll or fold. It has grommets for easy mounting.  At 21 in x 42 in it’s too large to fit in a suitcase, but could be boxed for airline travel. $217

100 W Semi-Flexible

Available directly from WindyNation

Two 30 W semi-Flexiblesunpower-30

You can get a pair of similar but smaller 30 W panels that will fit in a suitcase. $240 for a pair:

You would need a pair of “splitters” like these to join the two panels to the single wire for the Villager:

Less expensive are the “traditional” aluminum-glass framed panels

Five 15 W Solartec  solartec-villagerkit2

We sell a Kit which includes five panels, wiring, and mounting brackets, designed for permanent installation on a roof. The panels are only $63.

Two 40 W Monocrystalline40W-mono

A pair of these panels will actually fit inside a suitcase. They are heavier because they’re made of traditional glass cover in aluminum frame, but a nice feature is the plastic bumpers on the corners. It should be possible to join two together with a piano hinge for about $20, so they stay together but can be folded. $150 for two:

Most Expensive are the lightweight fold-able or roll-able thin-film panelsGSE-P3-62-flexPowerFilm60

Premium thin film panels cost over $10 per watt.

PowerFilm makes a 60-W panel that rolls down to a cylinder 26 in long and about 5 in in diameter, weighing 4 lb. Powerfilm R-60

Global Solar makes a 62 W folding panel, that folds down to the size of a notebook and weighs about 3 lb.  Global Solar P3-62  PowerFilm has a similar product.