To test this PSU outside of a computer case, you must simulate the "power-on" signal typically sent by the motherboard.
The HP D10-240P1A Go to product viewer dialog for this item.
Driven by an integrated PFC controller IC, a large power inductor (toroidal choke), a high-speed steering diode, and a primary bulk filtering electrolytic capacitor (usually rated around 420V / 150µF to 220µF).