The ratio of total facility power to IT equipment power, measuring data center efficiency. A PUE of 2.0 means half the electricity goes to cooling and overhead; 1.0 would mean perfect efficiency (impossible in practice). Modern air-cooled facilities achieve 1.4-1.6; advanced liquid-cooled facilities can reach 1.1-1.2. Lower PUE means less wasted energy.
Discussed in Chapter 3 of This Is Server Country