The number printed on a power supply is its sustained output — what it can deliver continuously without shutting down. It is not what your system draws, and it is not a quality rating. Two 750 W units can behave very differently under the same load.

Your actual requirement is the sum of what each part pulls at full tilt. In practice the graphics card and the processor account for the overwhelming majority; storage, fans and memory are rounding errors by comparison. This is why the honest way to size a supply is to add up the real components rather than reach for a round number.

The headroom question

Running a supply at 100% of its rating is legal but unwise. Efficiency peaks somewhere near half load, fans stay quieter with margin, and capacitors age faster when constantly stressed. Leaving meaningful headroom above your measured draw is the usual advice — enough that the unit spends its life in its comfortable band rather than at the edge of it.

Headroom also buys you your next upgrade. A graphics card swap is the most common change people make, and it is the change most likely to push a marginal supply past its limit.

Add up your parts in the power supply calculator and you get a figure based on your actual build instead of a guess.