Best battery backups/power solutions for non-removable batteries?

Currently split between getting a Zenfone 2 or the LG G4. Decision pending once some customer feedback hits the web after its release in about a week. One of the big dividing factors for me though is the battery situation with the Zenfone 2. It can't be removed, and by the looks of the teardown it isn't going to be possible to disassemble the phone and replace it in a year or so as it starts to lose its kick. That sort of leaves me with the LG G4 as the only current release phone with a removable battery and a microSD card slot, both features I absolutely love.

So what are some good suggestions for battery backups and the like that would pair well with the ZenFone 2? I've never used one before, but let's say I pick up a 10,000 mAh backup unit. How small can that be? Any recommendations? How long would it take to charge my phone off that? Any other suggestions for some way to manage without extra zip when I am a really heavy user? Thanks, guys!

I'm sure by now you've made a decision, but even if today you do not have one I would give this one a try:

4 ports, 5v 4.8 output, smart charging, and has dual input charge with an LED light to boot.
20,000 mAh so it will do you fine.