Will provided some more detailed information on the t540 power consumption:
[Back]Headless: With a fresh Ubuntu 24.04.2 LTS install, no extra services besides SSH and nothing plugged in besides power and a network cable:
Idle around 4.9-5.2W
Running prime95 and a Vulkan memory tester (to try to stress the GPU): max power usage was around 13.8-14.6WRunning K3s with three of them as control plane nodes and no worker nodes, they idle around 8-9W.
Adding a single display (2560x1440, just a Linux console) adds 2-2.5W of power consumption. I didn't test adding more displays, or doing anything else GPU related.
I haven't found many references to the option cards in the t540 documentation (like part numbers), but I did find some of the USB C ones on eBay with the part number L93688-001. With one of those cards installed along with a straight through JST SH 10-pin cable between the option card and the motherboard I can power the t540 directly with USB C PD (same power usage). For finding other parts the internal code name looks to be "Yellowfin".
One thing to note about the option cards compared to the more common FlexIO ones found on the ProDesk/EliteDesk series is that the thin client ones do *not* have an external cover attached. In other words, either you also find the corresponding replacement back panel with the cutout for your option card, or you're cutting out a hole from the back plate you already have. The thin client option cards also have different physical arrangements from the V1 and V2 FlexIO cards I had lying around, with no easy way to try to cram them in even if y wanted to.