1. Ensure it’s made to be charged entirely by the sun
No, we’re not joking. If you read the fine print on many solar chargers you will see things like “for best results, charge via a wall outlet” or “not intended to be charged entirely by the sun.” In order to keep costs down, some companies will utilize a solar panel that is too small for the unit. Read the fine print and get a unit made for the sun.
2. Right size your battery & solar panel
Batteries are rated in milliamp-hours (mAh). The higher the number, the higher the energy storage capacity. An average phone battery has a capacity of 2500-3000 mAh. If you plan on using 75% of its power for two days, a 5000 mAh battery would be sufficient (3000 mAh X .75 X 2 = 4,500 mAh).
The formula is: (phone battery size) X (% device battery usage) X (time) = power supply required
With time and usage, even the biggest battery will eventually be spent. Solar charging is crucial for emergencies, extended trips, and for multi-device power. The amount of energy that you get from a solar panel depends on certain variables:
-Size and efficiency of the solar panels
-Direct or indirect sun (including direction, shade, and clouds)
-Latitude and elevation
-Temperature and weather
Often, when you need a solar charger the only variable you control is the size of you solar panel, so it is important to size the solar panel appropriately. This gets technical but stick with us -- we break it down at the end -- or just skip to the In Summary for this section.
Solar panel sizes are measured in watts (w) and their power output is measured in watt-hours (Wh). A rough estimate of size can be made by multiplying the panel watts by the number of hours of sunlight, and factoring in any losses (we’ll generously assume 15% losses, which can come from inefficiencies in the panels, the charger, or the device you are charging). Under 5 hours of direct sunlight in the middle of summer, a 3-watt panel at about 85% production would produce 12.75 Wh (3w X 5hrs X 85% = 12.75 Wh).
The formula is: (panel size (w)) X (direct sunlight hours) X (%production) = output
In order to figure out how big (or how many watts) we need our solar panel to be, we can work backwards from our example above where we needed 2,250 mAh per day to keep our cell phone charged. To convert mAh to Wh, we multiply the battery’s milliamp-hours (mAh) by the volts of the solar battery and divide by 1,000. If we have a 5-volt battery this would be: (2,250 mAh X 5 V / 1000 = 11.25 Wh).
The formula is: [(mAh)*(V)]/1000 = Wh (or use this tool: https://milliamps-watts.appspot.com/)
To convert to watts, you reverse the calculation we used to get our solar panel output. Take the output you need (Wh), divide by the production (85%) then divide by the number of direct sun hours. This will give you the power rating (in watts) that you need for your solar panel. In our case above this would be (11.25Wh / .85 / 5 hrs = 2.65w).
The formula is: [(power needed in Wh) / (production%)] / (hours of daylight) = solar power rating
In summary, to enable you to use 75% of your cell phone battery capacity every day (with a 3000mAh battery) the solar panel on your charger should be at least 3 watts and the battery should have minimum 5000mAh capacity. A 3-watt solar panel will be roughly 30-40 square inches (195-260 square cm) depending on the type of panel, factors of efficiency and packaging. Sizing up the battery and/or the panels can give you flexibility and peace of mind.
3. Choose the right power output
Depending on the type of device(s), you may want to consider power output - or how fast a device can be charged. For this, you need to know the amps and volts of the charger. In general, the higher the amps, the faster the charge (some phones, like the iPhone, will only draw 1 amp, regardless of how many amps the charger has, but will not be harmed from a higher amp device). For a tablet, with a larger battery, you may want to consider a charger that has a 2 amp output to avoid extended charging times.
Some units may also have multiple outputs so that you can charge more than one device at a time. Most devices have shared circuitry where multiple devices will compete for the power. If you have a 2 amp circuit and are charging two cell phones pulling 1 amp each, this is not a problem. But if you don’t have enough power output, charging multiple devices (like two tablets) at the same time can significantly slow down the charging time for both units. In contrast, some devices have split circuitry, where each port has its own circuitry and can maximize the charging rate for each port.
4. Account for Safety and Longevity
There are two considerations that can help you understand how safe your device is:
With solar chargers or any battery pack, quality goes beyond the device itself. A low-quality product has the potential to harm the devices you are charging. Look for terms like “overcharge” protection that will protect the battery of your phone. “Overcurrent” protection is another important feature that limits the amount of power going in and out of the device to protect the circuitry on both devices. A good quality device will mention its safety features.
There are certifications that can help you assess the safety and performance of a device. CE and FCC certifications should be the bare minimum for any electronics. For solar lights, there is also the Lighting Global standard which is a World Bank initiative that supports sustainable growth of the international off-grid solar market by verifying the performance of solar lighting products.
5. Select other Key Features
Now that you have your basics covered, you may want to explore other key features, such as portability. If you plan to use the unit for backpacking or camping you want to ensure the unit is lightweight and transportable. Check to see if it can be secured to a pack while in transit so you can take advantage of daylight hours to power up the battery.
If you aren’t going to be mobile with your solar charger, then you may want to consider units that have separate solar panels and battery packs. This would allow you to permanently affix a solar panel to a roof, plug in the battery pack during the day for charging, and unplug it for use at night. These systems often have larger solar panels that do not require ease of transportation.
Lighting is another common feature on solar battery packs. A powerful light erases the need for a flashlight if you’re planning on using it for backpacking or camping and can be very useful in your emergency preparedness kit.
Some companies (like us here at Unite to Light) also have a social mission. For every one of our Solar Chargers you buy, we donate one to someone living without electricity or impacted by a disaster. We call it Buy One, Give One (BOGO).
The key to finding the right solar charger and battery pack is identifying how you’ll be using the charger and finding a product that fits those needs. Many units focus on supplying a larger battery instead of sufficient solar panels, but if you’re going to be off grid for multiple days, solar panels are crucial. From there, focus on quality to protect your devices and the charger itself. The right device provides the power you need, wherever you are.