Every step you take wastes energy.
If you use a solar panel to charge a battery, to use that battery to charge another battery, then use that battery to charge another device, you'll lose a minimum of 10% at each step.
- 100% of the solar energy goes into the Joos's own battery, so you are now are a maximum of 90%
- if you used that Joos own battery to recharge a Powercore 10000 then you'd then be at a maximum of 81% of the original solar energy
- If you then used that Powercore 10000 to then charge a device (say a mobile) then that mobile would hold at most 72% of the original solar energy
You then have speciifc to those Joos:
It looks physically very small solar panel. I'm seeing its specs at 8.58″ x 5.77″ and the solar panel is looking about half that, so I'm seeing about 25 sq.in, with the bezels at the top and bottom and the corner cut-offs. I don't frankly believe is is 18W, I'd say it would be doing well to be 10W, of which in even in good conditions you typically get 50% of that so I'm estimating 5W, so 5V at 1A. I own a 19W solar panel which has 3 9.5"x5" so it has 142sq.in so about 6 times the total solar panel area of the Joos, so I struggle to believe that something 1/6th the size of solar panels is capable of giving the same solar performance. Especially as my solar panel has about 21%-24% solar efficiency so Joos would have to in effect break the laws of physics.
Wow what a price. For its Wattage of solar probably 10W max (5W most commonly) you can buy a solar panel for a fraction of that cost. I see a price of $150. To put in perspective, most bigger solar panels in the 19W-21W range cost about $50-$60. It only has 5400mah and most 5000mah batteries are around $10, a 10000mah around $20, etc.
It is heavy, for what it is. It only contains 5400mah battery and for what looks to me at most 10W solar panel I see 24oz. Well the Anker 21W is about 16oz. A Powercore 10000 is about 6oz, combined you'd get be getting (guessing) at least double the solar energy for the same weight at the Joos, with double the stored capacity (10000 vs 5400 oz). What you'd be getting is when the sun is strong, able to take benefit of it of (guessing) at least double the speed, and then store double the energy for the same weight, and roughly for half the cost of the Joos.
Consider instead just having solar panel and just having a Powercore 10000. Say the Anker 21W (which was on discount earlier this week). It (as in reviews which measure it) gives about 10W so 2A @ 5V. You'd then have two main modes of use:
You plug in the Anker 21W directly into your device. You'd be feeding that device about 10W so that 2A is typically the maximum most devices can ingest (mobiles do well to be 2A, a smaller tablet is 2A and larger tables upto say 2.6A), so you'd be charging your device at the maximum speed (more so the smaller it is). You do this first. Then you're getting 90% of the solar energy into the device, not the 72% number I illustrate. You'd do this as your primary method when the physical conditions allow.
Once your device is charged, or say you are using the device and leaving the solar panel in a location like basecamp, then to not waste sunshine you plug in the Powercore 1000 to top it up. At times during the period the sun is strong and pointing at the panel, you'd be getting the 2A maximum output from the 21W Anker solar panel which is the maximum input, and you'd potentially fully recharge a drained 10000 is a day, more so if you can move the panel to keep at the sun. You can then use the Powercore 10000 to charge your device, in this case you are using 81% of the energy.
You mention you bought the Joos (cough) and you are going on a long bike ride. The Joos weighs the same as a 26800mah battery and would store days worth of what you'd get from the Joos's small solar panel. Consider simply leaving the Joos behind and buying just more Powercore. The Joos weighs about 3 Powercore 1000.