Your instance will only clone new content after you've federated with a community. And it's per community, not per instance.
It will also be generating thumbnails for websites that are linked, and a good chunk of the data requirement goes here.
I can tell you that, on average, my instance consumes about 700MB per day. I could cut that down if I federated with less communities, and I could get it down to 400-500MB per day (probably less) if I blocked my instance from generating thumbnails.
It's not a lot, but over time it will add up. My instance is pretty new, and I have no idea what pruning options are available yet. I've got over a month before I have to worry about storage space at the rate I'm using it.
As for system requirements, as long as you're not supporting users besides yourself, Lemmy will pretty much run on a potato.
TL;DR
400-700MB new data per day depending on your usage habits
Whatever you want to run it on will probably be fine
EDIT: Turns out ~90% of my Lemmy data is just for debugging and not needed:
That's it, I think some political people need their own potato! Put them on an island and the only contact they have is liking their own comments and posts..
Federation is done per community?! Ok, this creates more questions:
Then why are instances defederating whole instances if they're unhappy with the content of specific communities?
And more on topic: What about new communities? Do you need to manually federate with need communities on an instance? This seems like a hassle... How does e.g. lemmy-world do it?
Back in the day at least on Mastodon you can keep your instance unfederated, but I don't know about Lemmy, it would be good some Lemmy project admins advice here.
I am interested in this too. As far as I know, you don't store images or videos, so that should help keep it light. I can't image the requirements for storing a database and text to be high but I am no server admin.
Oh I am sorry, I thought I was on kbin where you need to upload them yourself and add a link. I see here on Beehaw you can store them. Sorry for the confusion
You store what is in your instance, and some content of another instances, according to your instance settings (in Friendica you can control what is stored in your instance, how much in megabytes for cache...), but in general you only need storage for content that you follow, and a bit more for public timeline content, but is not needed a copy of the fediverse in your instance.