Direct Solar Charging Test
An important part of a solar panel is how well it can convert sunlight into energy. Perfectly sunny days are often rare while outside but they're the best metric for testing the actual efficiency of a solar panel. To decide which model performed best, we tested each model under similar conditions, taking fastidious notes and comparing the results for each test.
When working with solar power, we learned, through trial and error, that the sun, wind, and clouds are constant environmental factors that we had to find ways to workaround. This is the beauty of solar power; it is a direct connection from our modern realm of electronic gadgets to the natural world. To get consistent data in our quest to find the best solar model, we had to narrow down our testing time so that we were sure that all were tested in the same conditions. We found that time on a sunny late April day. We laid out each of the models under peak daylight when there were clear skies and no clouds in Lake Tahoe. From 12:10 pm to 1:50 pm while temps were in the high 50s, we measured how much each device charged a 5mAh batter pack. We then compared the charges between the models and figured out which panels converted the most sunlight to energy.
By having stable conditions and a battery pack charger, we created consistent test results for the solar chargers.
Indirect Solar Charging Test
One of the biggest complaints of users is how small, portable models work on days that are less than "bluebird". If you spend a lot of time outside, you know that a perfect blue-sky day is elusive in a lot of places. Chances are there will be clouds running through the sky for some if not all of a day. Because of this, a solar panel that only performs in optimal conditions might not be the best option. To figure out which models did best in sup-prime conditions, we created a test that would compare each one's ability to recover a charge after being shaded. A cloud could pass overhead, a panel could close itself or fall over in the wind, or some other shadow could pass over the panel when left out for a few hours unattended. There are a million possibilities for the panel to not receive direct sunlight for the whole time it should be charging.
We wanted to make sure our bases were covered so we performed a test similar to our direct sunlight test. On a sunny early May day, we laid out each of the models under peak daylight when there were clear skies and no clouds in Lake Tahoe. For an hour and a half in the middle of the day, we measured how much each device charged a 5mAh batter pack. What we did differently was that we shaded the panels fully by placing a sheet 4 feet above the models and then we partially shaded them by placing a sheet and covering only half of the panel. We compared the model charges and found the conversion rates of sunlight to energy. Some were unable to continue charging the battery after the interruptions, while others continued to deliver charge with portions of the cells covered and even charged the battery pack a few percentage points more after returning to the sun after being shaded. Typically, the smaller watt contenders did worse than the larger wattage models.
Multiple Device Charging
An important ability of any solar charger is to provide power for more than one device at a time. Usually, when one device loses power, the rest follow so it's crucial that these solar panels can easily charge more than one device at a time.
This category applies to models with a 10W capacity or more. The smaller options just don't have the guns to charge two devices at once, as quite a bit of the output power is lost due to inefficiencies.
A few of the devices reviewed had more than two USB ports. We did not test beyond the two ports, however. Charging more than two devices at once can lead to a significant power drain, which isn't a problem for some of the models but to create a solid metric across the board, we stuck with two ports.
Portability
How well a solar panel can be moved around makes a big difference in terms of use. Weight gets high weighting in our scores; pun intended. After all, the whole point of a portable solar panel is to be, well, portable. For this category, we weighed each panel, measured it folded and unfolded, accounting for all the needed accessories, and then based our scores on an average person's backpack. A model that weighs less than a pound and is very compact is all we take for most outdoor situations: hiking, backpacking, biking, and climbing. If it weighs more than a pound and a half, it needs to do some heavy-duty charging of multiple devices and/or a laptop and is probably not ideal for carrying on a self-supported trip. If you are boating, setting up a basecamp, or traveling out of your van weight matters less. Take that into account when looking at the scores. As a general rule, the larger solar panels scored poorly, while the smaller battery packs generally got a higher score.
Consider that weight will increase if you need to bring multiple charging cables and/or a case. The weights in our spec sheet indicate the total weight of the setup we reviewed. Keep in mind that many panels work best if you charge it up at home and use the sun to keep it topped off on your trips off the grid.