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Lots of answers about use-cases of additional wifi networks, so I won’t go into that. I haven’t seen the downsides mentioned here, though. While technically you can run lots of wifi networks of off the same wifi router/ap, each SSID takes a bit of air time to broadcast. While this might sound rather insignificant since this is only a rather tiny bit of information transmitted, it is actually more significant than one might expect. For one the SSIDs are broadcast quite often, but also they are always transmitted at the lowest possible speed (meaning they require a lot more airtime than normal WiFi traffic would require for the same amount of data) for compatibility reasons. This is also the reason why it is a good idea to disable older wifi standards if not needed by legacy clients (such as 54 Mbit/s 802.11G wifi).

Having two networks is usually fine and doesn’t cause noticable performance degradation, having 4 or more networks is usually noticable, particularily in an already crowded area with lots of wifi networks.

For many li-ion laptop batteries, the manufacturer’s configuration of a 100 % charge is pretty much equivalent to overcharging. I’ve seen many laptops over the years with swollen batteries, almost all of them had been plugged in all the time, with the battery kept at 100 % charge.

As an electrical engineer you should know that technically there is no 100 % charge for batteries. A battery can more or less safely be charged up to to a certain voltage. The 100 % charge point is something the manufacturer can choose (of course within limits depending on cell chemistry). A manufacturer can choose a higher cell voltage than another to gain a little more capacity, at the cost of longterm reliability. There are manufacturers that choose a cell voltage of 4250 mV and while that’s possible and works okay if charged only occasionally, if plugged in all the time, this pretty much ensures killing the batteries rather quickly. I would certainly call that overcharging.

Since you already mentioned charging thresholds, I just want to say, anyone considering using a laptop as a server should absolutely make use of this feature and limit the maximum charge.

Compared to other SBCs, Raspberry Pis have been pretty inefficient for a while. A Pi 5 idles at about 3 W, which is pretty terrible for such a board, compared to other similar devices. You can get X86 PCs that idle at 3 W which are way more powerful. Other ARM SBCs use less than half that at idle and similarly less under load.

There are probably multiple reasons for that. The Pi’s SoCs have always used rather old process nodes, which are more power hungry than more modern ones used by other single board computers and PCs - 16 nm for the Pi 5 SoC and 28 nm for the Pi 4. Also, with the Pi 5 there is this additional “south bridge” chip which is attached via PCIe. This consumes additional power and for some reason the PCIe link is configured such that it never enters power saving states. I don’t know why.

Also, the power supply circuitry on the Pi 5 is far from ideal with its 5 V / 5 A power supply. Such a low voltage at such a high current can easily cause additional losses on the wire. That’s mostly relevant under high load though.

Since none of these require a Raspberry Pi to run, I would suggest using a mini PC (with an Intel N100 or similar) instead of a Pi 5. With all the accessories needed for the Pi, a mini PC can actually be cheaper and of course a lot more powerful. Since the Pi 5 is very power-inefficient, a mini PC can even be better in that regard too if that matters to you.

Especially for Jellyfin a PC with an Intel CPU with integrated GPU is awesome, since Jellyfin supports hardware transcoding with that.

Yes, the Odroid H series SBC probably come closest to OP’s requirements. Schematics are available on their website. They are also really low power with even the older H2 idling below 4 W.

Disableing the root login gains nothing in regarding security.

This is usually not the reason people recommend disabling root login. Since root is an anonymous account not tied to an actual person, in a corporate setting, you do not really know who used that account if you allow root login. If this is relevant for a personal home network is for you to decide. I would say there is not such a strong argument for it to be made in that setting.

Highly susceptible to replay and man in the middle attacks.

fwknop isn’t susceptible to either.

I was thinking about doing something similar and was considering running Android on a Raspberry Pi. There are unofficial LineageOS builds for the Raspberry Pi. I haven’t tried that yet, but I guess it should be possible to use the Jellyfin Android app on such a setup.

There is quite a significant difference. An ssh server - even when running on a non-default port - is easily detectable by scanning for it. With a properly configured Wireguard setup this is not the case. As someone scanning from the outside, it is impossible to tell if there is Wireguard listening or not, since it simply won’t send any reply to you if you don’t have the correct key. Since it uses UDP it isn’t even possible to tell if there is any service running on a given UDP port.

I always found the software updates of AVM - the manufacturer of those "Fritz!Box"es - to be of questionable quality. If you take a look at the source code that they have to release upon request of the GPL’ed source code they use, you’ll notice that they use ancient versions of the Linux kernel, Busybox and other tools. By ancient, I mean many years old, unsupported by upstream for years. Also, they only publish those sources manually when someone asks for them, which doesn’t bode well for their internal development processes. If they used CI/CD pipelines, they could easily push out updates of those sources with every new release…

Getting certs from Let’s Encrypt should work fine with any provider, even if you can’t open any ports, since they do support DNS challenge.

OP mentioned $0.40/kWh, so that would be about $17 per year with a 5 W difference.

Particularly in low-load scenarios there can be quite a big difference when it comes to PSU efficiency. While newer ATX PSUs have become better with regards to efficiency at low load, a Pico PSU can still be quite a bit better. Older ATX PSU often don’t even reach 60 % efficiency at 5 % load (which would be a typical load for such a system at idle), sometimes considerably less than that. At the same load a Pico PSU can easily be at 85 % efficiency.
Of course, at higher loads the difference is way smaller.

BirdNet-Pi is awesome. Highly recommended for anyone who likes birds. The BirdNet app for phones is also nice.
Btw, BirdNet-Pi also works fine on the non-plus Raspberry Pi 3.