I run my own instance, and that's not really how it works. While the instance is off, it isn't receiving new posts. So when you turn it on and log in, there will be nothing new for you to view. Everything you will see will be the same stuff as when you logged off last time. On top of that, while your instance is off, all the other instances that are federated with you will constantly be trying to communicate with your instance, causing all the other instances to do more work. Recently a change went into lemmy to automatically block instances that have reliability issues. And if other instance admins are watching their logs, they will probably block your instance anyway (lemmy.world has done this occasionally).

Your lemmy instance needs to have at least 98% uptime imo. If it goes down for a few days rarely, it's not a big deal. But shutting it down 80% of the time and only having it up while your using it will be bad for the lemmyverse and for you, the user.

A raspberry pi is inexpensive and is a much better solution, as it can be wired in all the time.