Overall, I liked Outdoor Gear Lab’s Solar Panel test. OGL tested a lot more panels than I did and they looked at more attributes of those panels. They tested for things like durability and ruggedness so I wouldn’t have to. IIRC OGL did their testing in an area of the desert SW that has better sun than SOCAL.

My goal was to take a small sample of panels and see how much actual power they made available through their respective USB ports measured in watts. Compare that output with the number in the panel’s name and see if it matched — in general, none were close. I’m not going to say which of my panel’s is best or even rank them, they all have their place. If I had to choose one, it would be the 21 watt panel simply for the power output under less than ideal conditions.

Testing was conducted using a pair of USB multimeters and a pair of Anker PowerCore 10000mAh power banks. I had both power banks at ~ <50% power. Then I read out the volts and amps from the multimeters. Since I intend to use the panels to charge a power bank while I have sun available this seemed like a valid testing method. Keeping the power bank(s) charged means you have power when you need it, even at night when the panels don’t work.

My only conclusion is that the watts in a panel’s name do not reflect the power output on a nice sunny SOCAL day at noon; in all cases the actual output was something less than their names implied. Clouds and low sun angle will further reduce the output. That’s the main reason the Renogy 21W came out well for me; it has enough reserve power output to charge a cellphone under less than ideal conditions. Don’t buy a 10 watt panel thinking you can get 2 amps out of its USB ports to charge your tablet, you’ll be disappointed.

One other point that applies to all the panels except the Renogy E.Flex5. In my opinion, in ideal conditions multiple USB output ports are nice to have if you need to charge multiple devices, but how often does that happen and does the panel have the total output to charge two cellphones in a timely manner? I’d much rather have a panel with streamlined electronics to reduce the power loss that comes from splitting the power between multiple ports. None of these panels has a lot of power to start with and it’s too bad they chose to squander it on multiple ports. OTOH, the Renogy E.Flex5 has a single USB port and it’s fairly rigid — no moving parts. It had the highest efficiency (percent output/watts in the name) of the panels I tested; still, it starts out small and the power out was limited to only 4.1 watts.

Along this line of thinking, one of my tests looked at the difference between charging devices using both ports versus charging using one port with the second port empty (thanks to haertig for this suggestion). When both of my power-banks drew power from the same port, the total output was slightly reduced from when both ports were used. So while one USB port had no current running through it, the other USB port had a significant increase, but the total output dropped — not a lot, but it did drop. This was one data point that led me to think there may be some efficiency to be gained in the electronics, probably at the expense of marketing (thinking 2 ports is better than 1 for sales)...

Note: Of the dual USB port panels, one has a pair of single USB ports on pigtails, while the others have a single dual USB port fixed to the panel usually in an enclosure (pocket). It’s nice to pull a USB port out of the panel’s pocket and hook it up, but the price is power. The dual USB port systems seemed to be more efficient than the 2x single USB port system; the dual port systems seem to split the power more efficiently between the 2 outputs.