Calculation of
Consumption.

Add the processor's TDP and the graphics card's board power — together they are typically 80% of the total — then allow roughly 50 to 80 W for the board, drives, memory and fans. The calculator does this for you from the parts you select.

Three reasons. Modern graphics cards draw brief spikes well above their rated power, and a supply running at its limit can shut down on one. Efficiency peaks near half load, so a unit that is always maxed out runs hotter and louder. And capacitors age: a supply delivers a little less after a few years than it did new.

No. A power supply draws what the components ask for, not what it is rated for. A 1000 W unit powering a 400 W machine consumes the same at the wall as a 600 W unit doing the same job — slightly less, in fact, because 40% load sits closer to its efficiency peak.

Only if your graphics card uses that connector. Cards that do will run from an adapter off an older supply, but a native cable is tidier and avoids the adapter becoming the weak point. Everything else works fine on a good ATX 2.x unit.

The efficiency difference is a few percent, so it rarely pays for itself in electricity alone. What you are usually buying with the higher rating is better components, a quieter fan curve and a longer warranty — which matters more, because the power supply is the one part that can take others with it when it fails.