Category Archives: Design Help

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.

Solar Charging of LiFePO4 batteries

Compared to lead-acid batteries Lithium Iron Phosphate  (abbreviated LiFePO4 or just LFP) batteries are simple to charge and quite forgiving about voltage set points. You can use most “12-V” lead-acid solar charge controllers to charge a 12.8 V LFP battery. On those with adjustable settings, here are our recommendations for voltage set-points and why.

Assumptions: Solar powered system in tropical or temperate zones, i.e. temperatures are always well above freezing, and charging stops every night when the sun goes down.

Significant differences lead-acid vs. LFP:

Lead-acid batteries need to be kept at their “absorb” voltage (14.1 V to 14.8 V depending on type) for several hours before they are fully charged. LFP batteries do not need an “absorb” time; they are ~98% charged when the voltage hits 14.0 V, and current in will drop to a trickle within a few minutes. It does not hurt them to be kept floated at between 13.7 V and 14.6 V for several hours a day, but it is not at all necessary and current will be minuscule. It is actually better for the total cycle life of LFP batteries if they DO NOT get a 100% full charge every day.

Lead-acid batteries need the charging voltages adjusted down slightly when the temperature gets above the design temperature of 25 °C. Some charge controllers have a temperature probe option and will compensate. LFP batteries do not need temperature compensation of charging voltages.

After reaching a full charge, lead-acid batteries with no load will naturally drop down to about 12.8 V or a little lower, whereas LFP batteries with no load will sit around 13.5 V or 13.6 V. The no-load voltage of LFP batteries won’t reach 12.8 until they are ~80% discharged.

Most solar charge controllers made for lead-acid batteries do three phase charging.

Phase 1 is “bulk” charging and restarts every morning. During this phase all of the power from the panels is dumped into the batteries. The current is limited only by what the panels can put out. This works the same for LFP or lead-acid

Phase 2 starts when the battery voltage rises to the “absorb voltage” (sometimes called “bulk voltage”). The charge controller will limit the current from the panels to keep the voltage from rising any higher. This “absorb voltage” is usually settable. The “flooded” setting is the highest voltage, usually 14.4 V or higher, which might possibly be used for LFP batteries if you have balancers installed, but beware, sometimes the “flooded” setting will do a weekly or monthly “equalize” at an even higher voltage, and this would be very bad for your LFP battery. If you lack a manual for your charge controller and don’t know for sure, do not choose the “flooded” setting on your charge controller. Instead, choose the “Gel” “AGM” or “sealed” setting (labeling varies). Any of those settings will have a lower absorb voltage somewhere between 14.1 V and 14.3 V and NO equalize cycle. They will work great with LFP batteries. If your battery doesn’t have cell balancers, it will probably last a little longer if you choose a lower voltage setting closer to 14.0 V, but it’s not at all critical.

Phase 2 (absorb cycle) ends when either a time limit is hit, or the charging current falls to some low number for a few minutes, depending on your charge controller’s manufacturer. Either method will work fine with LFP batteries. If you have the option of setting the absorb time, set it as low as possible.

Phase 3 Is usually a float cycle that will continue until the sun sets. A float voltage of anywhere between 13.6 V and 14.4 V will work fine with LFP batteries. If you have a choice, choose 13.7 V. This will keep the loads from drawing any current out of your battery as long as the sun is shining. Anything below 13.6 V will waste sunshine since it won’t pass energy to the loads until the battery is significantly discharged.

Equalize is BAD Some charge controllers can do a monthly automatic or manually initialized “equalize” cycle. This is only done for “flooded” lead-acid batteries, and raises the battery voltage above 15 V for a timed period, usually over an hour. This brings all the cells to a full charge balancing the cells, but also boils off some of the electrolyte. You have to add distilled water to the battery after equalizing. You must make sure equalize is disabled when using LFP batteries, it will damage them!

LVD Some charge controllers have a Low Voltage Disconnect (LVD) sometimes called low voltage cutoff (or your inverter will have a LVD voltage). It is critical for the health of your batteries to avoid excessive discharge, especially for LFP batteries. If you get below 2.0 V on any of the four cells, a LFP battery will be instantly ruined and no longer take a charge. Most manufacturers recommend a low voltage cutoff of no less than 10.5 V, but there is no advantage to you to set it this low. At 11.5 V the battery is 99% dead and you can only get another minute or two out of it before it drops to 10.5 V, so don’t set the LVD below 11.5 V if you have any control at all. When your battery gets old, and some cells are weaker than the others, it will last a lot longer if you haven’t been pushing the lower limit. We recommend you set the LVD to 12.0 V to 12.7 V. Higher is better for the battery. 12.0 V is around 95% depth of discharge (DOD.) At 12.7 V you will be cutting out at about 85% DOD and you will get many years of use out of your battery. Depending on the amperage of the loads and your wire size, you may need to adjust the LVD voltage down by several tenths to account for voltage drops in the wiring.

For all of the above reasons, if you have a charge controller with settable voltages like the Xantrex C12, we recommend the following settings:

For batteries with balancers:

  • HVD = 14.4 V (absorb voltage)
  • HVR = 13.7 V (float voltage)
  • LVR = 13.3 V (reconnect loads after a disconnect)
  • LVD = 12.7 V (disconnect loads when at 85% DOD)

For batteries without balancers (or with) a lower absorb voltage should prolong the life:

  • HVD = 14.0 V
  • HVR = 13.7 V
  • LVR = 13.3 V
  • LVD = 12.7 V

Safety considerations at battery end-of-life

When one of the cells in a LFP battery fails either from over-discharge, old age or a defect, the battery voltage will be low and remain low even when charging. A good charge controller designed for LFP will limit current to a trickle if the battery voltage is below 10.5 V, but controllers designed for lead-acid might not. The dead cell will get warm and build up pressure inside, causing the battery to visibly swell. If charging continues, it could pop the pressure relief valve and suddenly vent nasty fumes into the room. Users should be trained to watch out for low battery voltage (< 10.5 V) or battery swelling and not attempt to charge the battery if either occurs.

How Many Watthours?

When choosing a solar power system you need solar panels and batteries that can supply and store more than the average watthours you will be needing each day to run your equipment.
You may have heard of watts, but are wondering what a watthour is.
A watt (abbreviated “W”) is a rate of use. It’s how fast you’re using up the energy in your battery. You can use a few watts and run for a long time, or a lot of watts, and run for only a short while. The inverter or dc-dc converter has to be sized to handle the peak watts you will need, but the batteries and panels are sized to supply the watthours you need.
A watthour (abbreviated “Wh”) is a total amount of energy or work you can do. It is the watts used (rate) multiplied by the number of hours the device is turned on.

watts x hours = watthours (Wh)

The yellow bars in this document help you visualize the amount of power needed to run various devices.


Battery Rule of thumb: Your battery should hold 1 to 3 times the watthours your equipment uses per day.
A larger battery lets you keep working longer, through several overcast days in a row. You have to decide if you’re willing to pay for and lug around the larger battery, or just stop work during cloudy periods.
Batteries are often rated in ampere-hours (Ah) The watthours (Wh) are calculated as
Ah x volts = Wh
Below, the bar to the right represents the watthours stored in the listed solar battery system:System-CapacityWH
*The usable capacity of Lead-acid batteries is about half of their rated capacity.

Solar Panels

Panel Rule of thumb: On a sunny day your panels should produce more than double the watthours your equipment uses.

For a simple rough estimate: tabulate the daily watthours you’ll be using, multiply watthours by 0.5 to get the approximate panel size you need in “watts.”
The solar panels are the real source of your power, not the batteries, so you can’t skimp on these. In most tropical locations you can estimate the watthours produced by a solar panel on a sunny day to be about 4.5 x the rated watts of the panel. e.g. a “60 watt” panel will produce about 60 x 4.5 = 270 watthours of energy on a good day. (On an overcast day, only one 10th of that!) On a sunny day, you need the panels to produce enough watthours to not only run all your equipment, but also to charge up the system battery after a cloudy day.

Estimating you daily energy need

Portable solar power systems have very limited capacity so you must choose carefully the equipment you plan to run on them. Below, we give some estimates of what different equipment uses in watthours. Remember, everything depends on how long you leave things turned on.


Goal Zero 1W LED light1WGoalZero

If I have a 1 watt LED light, and I leave it turned on for 1 hour, it will use 1 watthour of energy. (1 watt x 1 hour) If I habitually leave it turned on for 4 hours every evening, it will use 4 Wh per day.

3W LED Spotlight3WLED

A 3 watt LED bulb illuminated for 4 hours will use 3 W x 4 h = 12 Wh per day. 12WHPendant Light

3W LED pendant light


Light bulbs are easy to calculate, because they always draw the same amount of power. Computers and printers are harder because their average power draw depends on what they are doing. You can read the watt rating on the power brick that comes with them, but that is the maximum they ever use, and is usually much higher than the average. Turning on your computer’s power saving features can cut the average power draw in half, so you should enable those features, even when it is plugged into the “mains” if you will be running on solar power.

Although laptop computers draw less power than desktop systems, there are large variations from one laptop model to the next.

A computer that draws an average power of 20 watts and is used for 8 hours a day will need 20 W x 8 h = 160 Wh per day.

If you’re planning on running on solar, choose your computer wisely. Here are some recent examples with my estimates of best case average power draw in translation work. I assume you use your computer exactly 8 hours each day.Laptops-WH

Note that in general older laptops will use considerably more power than the latest models using 4th or 5th generation Intel chips. For reference this is the 72 Wh Half-Pint battery:


Although the Half-Pint could run any of the above computers for a short while, many couldn’t run for a full 8-hour day. A “Villager” has triple the capacity of a Half-Pint and is more suitable for low powered laptop computers. A Power Hub can comfortably power two of the most power-hungry laptop computers, with energy to spare for lighting.

Picking a low powered computer

You can estimate the power draw of a laptop without owning and testing it yourself.

  1. Look for the average run time on battery as tested by reviewers. (for example 5.5 hours)
  2. Find the Wh size of the battery on the manufacturer’s site or in the reviews. (for example 68 Wh)
  3. Divide: 68 Wh ÷ 5.5 h = 12.4 watts average

This is the best you can expect if you keep the internal battery topped off. Recharging the internal battery is usually inefficient and will use up to 1.5 x more energy.


Ink-Jet printers use between 20 W and 35 W while printing, depending on the brand and model. Usually you can find this information on the manufacturer’s website and choose wisely. They use considerable power (4 W to 8 W) when idle and even use small amounts of power when turned “off,” so unplug them when not in use. You can print 100 to 200 pages in an hour.
Inkjet printing for one hour 20 Wh to 30 Wh 25WH
Laser printers are high powered inefficient devices. They average 325 W to 600 W while running, with spikes over 1500 W. Printing for one hour a day can use more power than running eight laptops all day long. Laser printers will not run on any of the small solar battery systems sold by GTIS, but we can design a custom setup to meet your needs.

BGAN Satellite Terminals

BGANs usually will be run off their internal batteries, and recharged periodically. Some can recharge directly from a solar panel. They use around 20 W while communicating, but typically you don’t need to use them for more than 30 minutes per day.
30 minutes of BGAN use per day 12 Wh 12WH

Projectors & Loudspeakers

Mini LED powered projectors can run on solar powered battery packs. The brighter ones will need more power. Here are the watthours needed for a 2-hour movie showing at maximum brightness.





If you’re showing to a crowd, you also need amplified speakers. For a 2-hour movie:


Smartphones, Tablets, Cameras etc.

You can read the Wh rating right off the battery of most devices, or calculate it from the milliamp-hour rating. (1000 mAh = 1.0 Ah.) Multiply Ah by the battery voltage (3.7 V for most lithium cells) to get watthours. The recharger is never 100% efficient, so you should multiply by about 1.2 to get a good estimate of the power needed. If you don’t plan on recharging your device every day, divide the watt hours needed by the number of days between recharges. Here are some estimates per recharge for various devices:



Fans draw more power than you might expect, and people usually want to run them all day long, so only the tiniest of fans can be used on a portable sized solar power system.
“O2 Cool” makes some small efficient fans designed to run on battery power. There is an 8-inch model, which draws 2.4 W on low and 3.2 W on high. Running it for 8 hours costs 19 Wh on low or 26 Wh on high. Their 10-inch fan will cost 40 Wh on low and 68 Wh on high for an 8 hour workday.
8 Hours of fan use:
O2 Cool 8-inch fan 19-26 Wh 25WH
O2 Cool 10-inch fan 40-68Wh 55wh