Why nameplate wattage is not enough
Most PSU calculators add up TDP figures and bolt on a fixed margin. That understates real draw in two specific ways, and both cause real failures.
Processors draw more than their rating. Intel's "processor base power" in particular is close to fiction under load — a chip rated at 125 W can sustain well over 200 W while gaming. We apply a per-architecture correction rather than trusting the sticker.
Graphics cards spike. Modern cards pull transient surges far above their board power rating for a few milliseconds at a time. Ampere and RDNA 2 cards are notorious for it. A supply sized to average draw can trip its own over-current protection and shut the machine down mid-game, even though every number on paper looked comfortable. That is why we size to peak, and why the recommendation is often higher than a simple sum would suggest.
Why we target around 60% load
Most supplies reach peak efficiency somewhere near half to two-thirds of their rating. Sizing there is not about saving electricity — the difference is pennies. It is about leaving genuine headroom for spikes, keeping the fan quieter, and being able to drop in a much faster graphics card in three years without buying a new supply.
Going much larger than that buys nothing. A 1200 W unit in a build drawing 300 W is not safer than a good 750 W unit; it is just more expensive and marginally less efficient at that load.
Quality matters more than wattage
The power supply is the one component whose failure can damage others. An 80 Plus Gold rating from a reputable manufacturer is a reasonable proxy for build quality, not just efficiency. A cheap 850 W unit is a worse buy than a good 650 W one for almost any build.