Most switched-mode power supplies (SMPS) for desktop PCs are AC to DC type. That means, SMPS takes AC mains input and outputs DC (3.3V to 12V). The DC outputs are used to drive various electronic circuits in the computer system including motherboard, hard disks, add-on cards, CPU, motors for fans, etc.
Power supplies are rated according to the maximum power (in watts) that they can produce. These parameters are normally specified by the manufacturers. Power output can be calculated if you know the rated output current and DC voltage output. For example, the SMPS specifications of a Traco power supply are listed in the table. Here, power output=voltage×current=24V×18.75A=450W.
It is important that your SMPS should be able to provide enough energy to drive all the devices in the PC at the same time. Most desktop PCs require power below 200 watts while running. But when you add external devices to your system, more power would be required. Also, more power is required initially when a system is switched on. Computer servers and workstations have lots of cards, drives, and adaptors, so power demand may increase beyond 350 watts, especially when the system is switched on.
In order to decide the SMPS power requirement, you need to consider the power of each device connected to the system and sum up their wattage to estimate the overall power requirement.