When choosing a DIN-rail 24 VDC power supply for an industrial computer, how many watts is enough?

Add up everything that draws power through the machine, then add headroom. 1. Maximum machine power: from the datasheet, often given as maximum current at a given voltage. If only the CPU TDP is listed, add about 15–30 W for the board, RAM and SSD. 2. Devices powered through the machine: PoE cameras, USB-powered devices, expansion modules, GPU cards. 3. Inrush current: peak current at switch-on is higher than running current. Example: a machine rated 60 W maximum, supplying PoE to 4 cameras at 12 W each (48 W), totals 108 W. Adding 30% gives about 140 W, so choose a 150 W or 240 W supply. Two more notes: DIN-rail supplies usually must be derated when the cabinet exceeds about 50–60 °C, so check the derating curve in the supply's datasheet; and account for heat in the cabinet, since almost all of the computer's power ends up as heat.