Many low-cost airlines do not have power ports at the seats, so you may need to take an external battery for your smart phone and tablet if you plan to use them for in-flight entertainment. This external battery will do the job, and it costs only $9.20 from Amazon. Shipping is free if you have Prime.
The price shown at Amazon is $13.99, but that will drop when you use Promo code LHISM4IX
Note that Amazon prices change very frequently.
I have one of these batteries, and have used it for about a year without problems. Someone once discovered that, sometimes, you get more power out of the 1-amp USB port than out of the 2.1 amp one.
This battery weighs 344 grams (12 ounces, the same as a can of Coke), but the internals are metal so it feels heavy.