I tried out Kiwix this week.

It is a library manager for offline content.

A thousand downloadable ‘books’(1000s of GB) from Wikipedia, Stack Exchange topics, ebooks, kids books, prepper content, to the Blender Open Movies.

This is the easiest way to setup an offline library of quality content.

(It is not in FDroid, but installs via Obtainium just fine)

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Maybe look at AARD2 as an offline Wikipedia browser too.

If you are so concerned about battery life that you are looking to jailbreak a phone, instead you should look into a power solution such as power banks - or even better, a solar panel with a controller that stores to a 12 volt car battery or some such.

In our off-grid place, we have cards, dice, and physical books.

Movies will eat battery no matter what.

Make sure your device and the OS support hardware decoding - since you want to transcode them to 720p anyhow, I’d say h264 is your best bet. Don’t try to compress them too much; my guess is that battery will be more expensive than storage in your off-grid scenario?

FWIW I recommend going Linux for this sort of stuff. You simply have more control. It would be cool if there’s a phone that supports full Linux; maybe check PostmarketOS’ supported devices.

I recently thought about repurposing a TV box or whatever they’re called - it’s possible to install Linux on some of them. Maybe check out Armbian. Of course you’d still need a screen then.

In the interest of battery life and redundancy, I think it might make sense to have 3 devices.

Ereader with an e ink display for reading, a lot of these can last days or weeks on a charge easily

An mp3 player for music. I don’t know what the current state of mp3 players is, I suspect a lot of no-name imported garbage, but over a decade ago I know my iPod used to go days or weeks on a charge with pretty heavy usage. Probably look for whatever has the least bells and whistles you can find- no touch screen, physical controls, etc. if you’re up for a bit of tinkering I’m pretty sure there’s a pretty active scene for people modding old iPods with better batteries, more storage, etc. that would probably be a great option.

A tablet or smartphone for movies, or possibly a laptop (I’m not an apple guy, but I’ve heard MacBooks have pretty insane battery life these days.) Keep all the wifi/cellular/Bluetooth/gps, etc. turned off, keep it on power save mode, disable anything you can that you don’t need to watch movies. Unfortunately if such a thing as a dedicated video-only tablet exists, I couldn’t find it with a quick search. If such a thing can be found, I’d probably recommend that.

A dedicated device that does one job well will usually be more efficient at that thing than a multipurpose device like a tablet, smartphone, or computer that needs to be able to do it all. An mp3 player only needs to be able to play music, it doesn’t need to be running a full-on OS that’s capable of sending emails, making phone calls, playing games, etc.

Also that way if one of those things does die on you, you still have the other 2.

I saw in one of your other comments your concern about a tablet having a bigger screen would be a bigger drain on battery life. That’s true to an extent, bigger screens draw more power, but since the whole device is bigger they can compensate with a bigger battery. I haven’t exactly done an exhaustive survey of tablet battery life and don’t care to look into it, but in my (fairly limited) experience, they usually pretty much at least break even or surpass phones in battery life. I have a cheap tablet that I really only use for reading it lives in my bag, usually in my car, often forgotten about for days or occasionally weeks at a time, and doesn’t exactly get heavy usage, but it usually can go at least a few days without a change, even with WiFi and Bluetooth left on. If I’m not using it at all, it can sometimes go a couple weeks just sitting idle. It’s usually good for at least a couple hours of streaming HD video, with WiFi turned off and 720p video on internal storage I imagine it’s good for at least a couple movies.

WiFi and cellular data are pretty big power drains too. I know when I check my battery usage on my phone that probably accounts for about ⅓ or so of it. Having those turned off can go a long way. Jailbreaking/rooting your phone to disable unnecessary services probably wouldn’t hurt, but that’s probably a drop in the bucket compared to just keeping your device offline.

Maybe an old tablet? (They usually have larger screens and larger batteries)

I don’t think jailbreaking will change a lot power consumption. Maybe even the opposite, as you may break service that aims to decrease consumption.

On my stock android (cross call core X4), in airplane and battery saver mode, it can last for a week. I use it when trekking and just use GPS with osmand from time to time.

Just be sure to disable all shitty apps (WhatsApp, gmail, etc…)

I might consider kodi since it will probably be more organized. Vlc will have normal file navigation but with kodi it will organize everything into movies and TV shows. If its going to be a purely offline device then you can just flash a custom ROM and see a decent amount of savings. It will likely be difficult to find a current generation phone with an sdcard and a tablet might make more sense for this use case. If you know you are gonna flash the device you are going to get you should make sure it has decent custom ROM support. Generally if it has an official lineageos build then it is well supported. All of this might honestly be more trouble than it is worth if you are not experienced with flashing a custom ROM so maybe just start with stock and see if you actually need to first.