I would look into thin clients and Lenovo etc. tiny PC for office on eBay. I run old low power low noise rackmount Supermicros which are nice but hard to find at low prices.

Factor in power bills and heat and noise into your calculations.

Also opnsense, but on thin client.

Why not postfix?

I hear you, but Proxmox does a great many more things than just run containers. Admittedly, many selfhosters won’t need these.

It’s a NUC so sufficiently poweful. Proxmox isn’t fat by any means. If you run your stuff in containers then Proxmox (I aways install it on top of Debian) is your hypervisor is your base system. You typically don’t install stuff on your hypervisor, though I do some very select things.

Proxmox with Debian containers.

You’re probably drawing about 400-450 W.

My current supplier rate is about 0.6 EUR/kWh. I make some 1/2 to 2/3 of my power myself, for a price that’s less than half of that.

How many W are you pulling, on the average? Or kWh per year.