Disregard the arm-waving guesses at how much power the PC "might" draw, and buy a Kill-a-Watt. Bonus points if you get a better type of power monitor that can data-log.

The Kill-a-Watt can also tell you a lot about power factor. Low power factor means it is drawing more actual current than useful current, which is an artifact of using really cheap power supplies.

With hard data, you can troubleshoot exactly when a PC is drawing what power - and if that's where the problem lies, you can iterate on changing the PCs' draws or improving their power factor.

For instance, this could be as simple as upgrading power supplies, or flipping a BIOS setting that tells it to spin up hard drives one at a time, instead of all at once.

Disregard the arm-waving guesses at how much power the PC "might" draw, and buy a Kill-a-Watt. Bonus points if you get a better type of power monitor that can data-log.

The Kill-a-Watt can also tell you a lot about power factor. Low power factor means it is causing interesting side-effects that masquerade as drawing more current than you actually are on average. Having high power factor of 99% is not a problem with a little bit of extra engineering, but it is not a priority for all builders, least of all the cheap ones.

With hard data, you can troubleshoot exactly when a PC is drawing what power - and if that's where the problem lies, you can iterate on changing the PCs' draws or improving their power factor.

For instance, this could be as simple as upgrading power supplies, or flipping a BIOS setting that tells it to spin up hard drives one at a time, instead of all at once.