Powerbank

Second question first: "How much you need" is something only you can answer - and depends on what exactly what you are hoping to power, and for how long.

First question: Are there restrictions? Yes, there absolutely are. Unfortunately, the limits may not be enforced consistently everywhere. In the USA, the FAA-imposed limit on batteries (that's what a "powerbank" is) rated at 100 watt hours or less (over 100 Wh would require prior approval from your airline, which is a hassle to get and while theoretically possible, no way I would bother). The good news is that 100 Wh is a pretty big one (but there are larger sizes). Yours should have its rating written or stamped or etched on the outside. I've got one here that's rated at 96 Wh and 26800 mAh (just barely below the limit) and it's a beast - measures about 7" long X 3" wide X 3/4" thick. I've used it to power several phones and tablets on world-spanning trips and it carries a lot of juice (about $66 on Amazon - here). Great unit. Note: all of these things are surprisingly heavy (batteries, you know?), so don't go overboard, you will need to carry them. You'll also need a charger block (wall-wart) and cables.

Other countries have different limits. My understanding is that China has a lower limit and is also among the most strict and intrusive when it comes to enforcement.

For most consumer powerbanks (small-ish sizes) you will be fine. You will typically need to pull it out when going through airport security inspections (considered "electronics").

Also keep in mind that USB power ports are increasingly common in airports, on planes and elsewhere, so you may not really need mobile power sources as much as you think. And then there's the notion that you are going there to actually experience the place, so it may not be a bad thing that lack of power might cause someone to stop looking at a screen and look up at the real world for a while (there have been times when this was actually a blessing, though the owner of the screen didn't feel the same way at the time).

To clarify, all airlines can refused to carry Dangerous Goods, regardless of whether cargo or in the passenger cabin, whether or not the customer has complied with IATA DG, etc. I do logistics for a living and can only get DG on China Airlines out of Bangladesh, for example.

I have a Blackweb 7x charger, clocks in about 96 wh as well, and I know that with United I would be fine. You'd have to google your airline and extra battery or battery pack to get their in cabin limits (these are generally prohibited in checked luggage).

Yes, I would bring one, but not a huge heavy one. I used my iphone7 mostly for pictures, maps, some RS walks. A few days my battery ran low, and I plugged it into my little lipstick sized power bank with 5000mAh. I’m not one to spend all day on my phone, but it was nice to know that if my phone did die and I needed to look up a map or train schedule, I had a backup.

Mike,

One other point to mention is that the power bank most likely uses a Lithium-Ion battery, and those must be packed in carry-on luggage and not checked luggage.

It's not all that difficult to use your existing wall chargers. You will need a converter plug to fit EU outlets & you may or may not need a 220V to 110V transformer. Read the fine print on your chargers. Both are cheap.

In terms of capacity, what exactly are you looking to charge? There are small powerbanks (the size of a few credit cards stacked up) that can charge a phone a half time. In other words, they're just to keep you from getting cut off on a call, until you can get your phone to a charging point. These are light and can be carried easily. On the other end are ones like the one David linked, which can charge a tablet several times over, and often have multiple outputs to charge several devices at once. As David said, these can be not only large but heavy.

One other consideration is the charging standards your phone uses. If you have a newer and higher end phone, it is probably compatible with quicker charging standards, but you'll need a compatible powerbank to get these quicker charge rates. Newer and higher end phones can also charge wirelessly. And if your newer phone is an Android model, it's likely to have a USB-C charging port rather than a micro USB port, while Apple products use the Lightening connector. If you do get a micro USB charger and need to charge a USB C device, you can buy adapter plugs very cheaply (that's what I did rather than buy a new powerbank, but of course I don't get quick charging or wireless charging with it).

One online rating source for gadgets is The Wirecutter. Here's their listings for Qi wireless chargers: https://thewirecutter.com/reviews/best-qi-wireless-charging-power-banks/

for USB-C chargers: https://thewirecutter.com/reviews/best-usb-c-battery-packs-and-power-banks/

and for regular USB chargers: https://thewirecutter.com/reviews/best-usb-battery-packs/

So it looks like most of the power banks being sold are under the safety limit.

We will be carry this on as we normally don't check any bags.

Looking to power up 2 iPhones and 2 iPads, all of recent vintage.

Thanks for all the feedback.

make sure that there is a 2.1 A USB socket on it of you want to feed iPads.

Some of the small ones will only power phones.
You don't want to blow up your iPad or the power bank by using it with the wrong device.

I usually find while out and abOut that it's really only my phone that I need to recharge. My iPad I’m not using as much during the day so can just recharge it in the evening in the hotel room. So I usually just bring a “lipstick” style charger. Or you could go for one of the credit card ones. Really, you don't want to go too big, or you won't want to haul it around. The most effective charger is the one that’s with you.