Charge Interruption Recovery
One of the biggest complaints of solar panel users is how terribly small, portable panels 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. Because of this, a solar panel that only performs in optimal conditions might not be the best option.
To figure out which panels did best in sup-prime conditions, we created a test that would compare each panel's ability to recover a charge after being shaded. A cloud could pass over head, a panel could close its self or fall over in wind, or some other shadow could pass over the panel when left out for a few hours unattended.
We wanted to make sure our bases were covered, so we let each panel charge a battery for 10 minutes in full sun. Then measured the output with our multi-meter. Then, we shaded the panel intermittently for the next 10 minutes and measured its output after being shaded. Some panels 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 sun after being shaded. The smaller watt panels did worse than the larger wattage panels, which made sense. The Renogy 5W was our lowest scoring panel for this test and the Anker 21W performed the best.
Phone Charging Speed
When working with solar power we learned, through trial and error, the sun, wind, and clouds are constant environmental factors that we had to find ways to work around. 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 a consistent data in our quest to find the best solar panel, we had to narrow down our testing time so that we were sure that all the panels were tested in the same conditions.
So, to answer the popular question of how long will this panel take to charge my iPhone (or Android, etc.), we had to narrow down our testing period to see how much charge an iPhone 6 got after plugging into each panel for 10 minutes. We fully charged phones with the panels as well, but to get a side by side comparison, the 10 minute window proved to be a good limiting factor and gave us a range of numbers as the panels worked to charge the phones.
Multiple Device Charging
This category applies to panels with a 10W capacity or more. The smaller panels just don't have the guns to charge two devices at once, as quite a bit of the panel's output power is lost due to inefficiencies. This means that the Renogy 5W and the Outad 7W are out of the running for this category. The Goal Zero Nomad 7 is bulky for its size, but does come with multiple options for charging, including a converted to plug into your car.
Of the panels that do have this capability, the Anker 21W performed the best. It was able to deliver a charge to two external batteries consistently. In contrast, other panels in this test would appear to be charging, but would actually never increase the charge of the batteries.
From this test, we did learn that it drastically decreases the efficiency of the panel to have two devices plugged in at once. Because they all have a relatively small wattage, its best to have an external battery or two so you can charge off of those, rather than relying fully on the panel for all you needs.
Out of the box, all nine of the panels we tested this year appear to be quite similar. They are all black, with heavy-duty canvas fabric on the outside, with some kind of pouch to store gadgets and cords. The panels themselves are all monocrystalline in composition, but have different coatings and additions depending on price. As we scoured the internet doing research for this review, we found many complaints among customers about the durability of the panels they purchased. Because of this, we decided to pay particular attention to the way each panel held up over the testing period.
Here are some of the questions we focused on when assessing the durability of the panels. Are they delaminating? Does the velcro continue to stick? Are the panels warping or melting? Does the panel still work after a month of use? Two months of use?
Weight and Portability
Weight gets high weighting in our scores, pun intended. After all, the whole point of a portable solar panel is to be, well, portable. A panel that weighs less than a pound and is very compact is all we take for most outdoor situations: hiking, backpacking, biking, and climbing. If the panel 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, weight doesn't matter. Take that into account when looking at the scores.
Also, consider that weight will increase if you need to bring multiple charging cables and/or a case. The weights in our spec sheet will indicate total weight of the setup we reviewed.