I bought my battery bank from ebay. It is a 500000mAh Portable Solar Panel Dual USB External Battery Power Bank Pack Charger. I doubled the USD price in my comments because I suspect people will want something that charges quicker than what I need, but from my testing, I'm delighted.

You'll need to decide how many devices you'll want to charge each day before you can decide what sort of battery bank will suit you. If you're only charging an MP3 player, a smartphone, some speakers and lights, then the one I mentioned will probably just be enough.

Sadly, battery banks are something that you'll need to research for yourself. I can't even recommend a range of brands because this is the first I've bought and I can't compare it to it's competition.

Most battery banks will charge even under overcast weather. All can be charged quickly from a live USB source if you know power shortages are coming. My power battery bank instructions said to run it until flat before charging it to full... to ensure maximum battery life.

Just leave them on the windowsill that gets the most sun and, if you bought one with enough capacity and one with solar cells with enough quality then you'll be able to recharge all of your devices whenever you need to without the bank running flat.

As for the pool chlorine, anything will do that is just the cheap pool chlorine powder just as long as it has no other active ingredients or brags about additional weird shit that you don't want to digest such as blue coloring or sea salt or other fancy shit.

And don't fear using chlorine because chlorinated water is what comes out of the tap. Most people have no idea that chlorine is what our water treatment services use to keep the water safe... **JUST READ THE DOSAGES CAREFULLY because if you're chlorinating a small container such as a 5 gallon jerry can then you'll need a ridiculously small amount. I'd recommend figuring out a fool-proof way to measure enough pool chlorine powder to treat whatever containers you have in advance, without relying on devices like electric scales which will eventually run out of batteries or break.

Also, if you're interested in prepping for interruptions to the water infrastructure, I'd recommend finding who sells second-hand 52 gallon pickle containers in your area - they usually cost about $20 USD. They are huge, tough, plastic containers about 22 inch diameter and 40 inches high with massive screw-on lids. Clean them out with washing detergent, fill 'em up with tap water and seal the lid. It doesn't matter if they sit in the sun for 20 years because you'll chlorinate that water when you need it. Also, don't bother drilling a tap into them because that only risks leakage if the seals degrade - instead just open the screw-on lid and lower a length of hose until it touches the bottle, block the end with your thumb, withdraw the hose a little so you're not siphoning silt... and water will flow.

52 gallons of water will last an adult for 100 days without rationing.

You can also do other stuff like disconnecting a down-pipe and collecting rainwater on your roof into buckets to top up your pickle container. Avoid the first few hours of the first rains after a dry-spell because it will be full of whatever dust and crud collected on your roof since the last downpour.

If you have any questions then please ask.