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Power usage effectiveness (PUE) has become the default way we discuss the overall efficiency of data centers. Regulators cite it, sustainability reports lead with it, and operators compete to push it closer to the theoretical ideal of 1.0.

It's a useful number, but it’s also an incomplete one. PUE measures everything that happens to power before it reaches the server — cooling, lighting, and power distribution — and stops the moment power crosses the server's threshold. What happens after that point, as electricity moves through the server itself and down to the processor, sits entirely outside the metric. A facility can report an excellent PUE while still losing a meaningful share of its power inside the server invisibly.