Love me some solar power!I have to admit, I am somewhat skeptical of the claimed efficiency of better than 21%, making it one of the highest efficiency solar panels available in the mass market at this time. It is possible that it is true, but considering the converter that is taking the electricity from the cells and making USB standard power *has to* have some loss associated with it, it seems unlikely.Being a tech minded guy, I test stuff. Its what I do.As everyone will tell you, if using a solar charger, it is better to use a battery pack to store the power when the sun is shining and charge your devices from the battery pack. This solar charger is not a battery pack, it simply takes sunshine and creates 5V output through standard USB ports.**Update** 4/25/15Bright sunny day today, measures around 165,000 lux (using my Galaxy S5 and "GPS Status" app on phone) just past solar noon in southern new england.Today I used 2 Drok USB power meters, one in each output port and two devices (phone and battery pack) to pull maximum power from the panel.I was pleasantly surprised to see a total charge power of 1.9 Amps!! (divided between my battery pack and phone)This is a significant increase in power 'generation' from the last test, and is almost as powerful as the fastest USB wall chargers available today, so to me, this is huge! It still falls about 50% short of the 20W claim but as previously noted, solar panels never hit their claimed power production numbers due to a standard which is laboratory based and effectively impossible to achieve in the real world. Additionally, there are conversion losses to be accounted for with this product.All in all, the X-Dragon High Efficiency 20W Solar Panel is a pretty awesome way to have real, usable USB power anywhere the sun shines.Highly recommended!** UPDATE ** 3/16/2015Bright sunny sky today, measures at around 110,000 lux, but still, ides of March sun @ ~41°N LatitudeInitially I hooked up two high drain devices to see what the maximum output would be but it never got higher than it did with one high drain device, specifically, my Ravpower battery pack for portable charging of any usb powered device.I saw a peak power output of 1.37A @ 4.91V for a total of 6.72Watts output power. On one hand this is disappointing considering the stated 20Watt ability of the solar charger, BUT considering that spec is STC (standard testing conditions) no solar panel ever produces the stated STC spec in the real world.Using my phone as a point of reference, the battery in my phone is 10.8Watt hour capacity, charging at the measured value of 6.72watts *Should* take about an 1:40 (one hour forty minutes) to charge fully from dead.. Which is about what it takes when plugged into an AC powered adapter.. so I am completely satisfied with the purchase at this time.Conclusion, it does the job it is supposed to do, creates free power from the sun to charge your USB powered devicesOriginal test 3/15/2015**TO BE FAIR, conditions are not ideal for testing today, being PRETTY OVERCAST at the moment and ~41°N latitude.Using an app on my phone (Galaxy S5) , I measured the currently available light at around 11,000 lux.I set up the panel angled somewhat to the south, after a couple seconds the LED lit up on the power outlet so I plugged in my Portapow USB power meter in line, and plugged in my Ravpower battery pack as a load to take the charge power.The meter indicated 4.93V @ 0.34A which means it would probably charge your phone, but fairly slowly.Most plug in USB wall chargers produce between 1 and 2.4A at 5v so by comparison this is a fairly slow charge.The battery on my phone is 2800mAh so at a .33A charge rate it would take 8.2 hours at current conditions to fully charge the battery from dead.. if the phone was off.Bottom line, in overcast conditions the power production is fairly pitiful, but that was somewhat expected.This review may be updated again when/if I am able to test the unit under even better conditions.