The PicoPSU is a passive power supply for ATX machines, which (usually) requires an input voltage of 12V DC. Due to its design it's more efficient than the old, noisy ones; plus it places most of the heat generated outside of the chassis resulting a much more cool, stable system to work with.
The unit comes with strictly basic accessories: one motherboard cable (with a MOLEX, a SATA and a floppy drive connector) and one power input cable. The first thing I had to do is to make a 1 female - 4 male MOLEX splitter, since the PicoPSU has only one of this and the backplane of the MicroServer gets the power from 4 connectors behind the HP logo. Stripping down an old AT power supply left me with more than enough parts, only a little bit of soldering and assembly was required. Remove the stock PSU by undoing the 3 screws on the back and simply sliding it out to the front. The next step was to find a location for the PCB in the chassis. Unfortunately the motherboard cable was really short, so I had to "leave it hanging" at the place of the expansion cards - fortunately I have none. To make sure nothing will short anything out I cut a piece of plastic and attached it to the PCB. The last thing to do was to cut a small sheet of metal to cover the holes left on the back.
As a last, cosmetic action I cut the unnecessary connectors off, leaving myself more space to tuck everything back in the tight box.
Fitting a PicoPSU is basically a old-out-new-in, but expect challenges during the process. It took me about 2 hours from shutdown to startup; including mid-phase tests and making the small cover as well. As for the sizing I advise to measure your needs before ordering one: with 3 HDDs and 1 SSD; without any expansion cards the PicoPSU sucked less than 5 Amps, meaning a total of 60 Watts. Since my power brick is 120W and the PCB can handle 180W I think I clearly oversized everything - at least I have enough room to grow