The HP ProDesk 600 G6 SFF locks you into its factory PSU

HP’s G6 SFF keeps the same proprietary PSU arrangement as its predecessor — a 180W or 260W 80 PLUS Gold unit that’s built into the chassis and not user-serviceable. It’s fine for the stock config and maybe a low-power GPU, but you’re not squeezing any extra wattage out of it down the road. Order what you need the first time.