Dell OptiPlex board options available.
Dell OptiPlex board options available.
In practice it's around 75W, though since most PSUs run at much lower power, I'd skip the highest rating. What are you planning to install?
Two points to note: 1) When replying, include quotes or mention them so the other person sees your reply. 2) The RX 550 is a 50W GPU, which should work, but make sure you use a low-profile card for compatibility.
Warning ahead, my 2015 OptiPlex struggled with power demands above 35 watts in PCIe 16x slots. It would freeze under moderate GPU load, lose power from USB ports, and experienced various oddities until I replaced the PSU and swapped the GPU for one with proper power connectors. A similar issue appeared with a slightly older model, though it ran closer to 50+ watts.