Planning of the network cabinet and its contents

What does the UPS have to do with the fuse box? A UPS protects against the following cases:

[*]Power outage
[*]Undervoltage
[*]Overvoltage
[*]Frequency changes
[*]Harmonics

Your fuse box certainly can't do that....

A small UPS? For what.. my patch panel hangs above the fuse box, the Speedbox stands on it .. next to it, the electricians installed a double socket for me. I think it looks like that in 95% of all home builders' cases as well

What others do doesn't matter, but the tip remains - if you want to do it properly - to use a small UPS. It absorbs surges and protects the hardware behind it. In some new buildings, the entire home automation is connected to a UPS. Not entirely pointless, but nobody really needs it. If we discuss it that way, I would already be out with the 24-port patch panel, because most don't need that anymore either. But we're here at "how do I do it best"

Exactly, we now need a 24-port patch panel because 20 connections are installed throughout the house. However, I am still concerned about the size of the cabinet and any possible things I might still be overlooking.....

If you have enough space in the room, just take a few more U [HE] units, the price difference is really only marginal, then you are on the safe side.

I don't need a UPS in the network cabinet, my E3/DC storage seamlessly takes over this function in case of emergency....is there anything else that is often forgotten? Thanks for the tip about 1m....

Although "seamlessly" is not quite accurate, the E3/DC storage has an emergency power function but is not a UPS, so it takes a few seconds before it takes over the supply in the house. During that time, the power is also off. A UPS could bridge this time, for example, without the modem, router, switch, server, home automation, etc. possibly having to restart/reboot/etc. and potentially getting stuck in the wrong status/etc.

But honestly, this is nitpicking on a high level; an emergency power function is certainly a nice feature in case of emergency. Neither is decisive in a critical situation and I would consider both just a nice-to-have.

For the equipment, I would choose 12-15 U, smaller only in absolute exceptional cases (if the room does not allow it). Otherwise, everything has been said. You have enough space behind the panel for proper cable management and can easily store excess length. Of course, don't put in 3m loops per cable. Otherwise, that should work. Shelf, switch, possibly POE injectors, router, UPS. For your network drives, you just have to see how high they are and that they can be easily removed if necessary. If an HDD is defective, you want to replace it without tearing everything apart.