Why your PSU needs headroom: GPU power spikes, peak CPU draw, and the 30% rule of thumb

A lot of PC builders size their power supply like this: add the CPU's TDP to the GPU's TDP, add a bit for "the rest", and buy the nearest wattage. That usually boots. The trouble shows up later, as a random black screen or reboot in the middle of a demanding game, while temperatures look fine and nothing in the logs explains it. Very often the cause is not average power. It is peak power. Average draw vs. what the PSU actually sees A monitoring tool that reports "GPU power: 220 W" i