# Power Usage Effectiveness Power usage effectiveness is a single number: total facility power divided by the power delivered to computing equipment. A facility drawing 150 megawatts to run 100 megawatts of servers has a figure of 1.5; one drawing 110 to run 100 has 1.1. It is the industry's standard efficiency measure, it has driven enormous real improvement, and it is worth understanding its limits as clearly as its value. > [!note] The one-sentence version > Everything the building consumes, divided by everything the computers consume; the closer to one, the less overhead. ## Why it exists In the 2000s a great deal of data center power went to cooling and conversion rather than computing, and nobody could compare facilities. One ratio made overhead visible and comparable, and visibility produced a decade of improvement: containment, warmer supply temperatures, free cooling, efficient conversion, and variable-speed everything. Figures that were commonly above 2.0 are now routinely near 1.2 and better in well-designed facilities. Few metrics in engineering have earned their keep so plainly. ## What it includes The numerator is everything crossing the meter: cooling plant, pumps, fans, lighting, power conversion losses, and the computing load. The denominator is the power reaching the computing equipment. Overhead therefore means cooling and electrical losses, and a facility improves its figure by reducing either. ## What it hides - **Climate.** A facility in a cool dry climate reaches a good figure with less effort than one in a hot humid climate. The number compares designs and locations together, and location often dominates. - **The water trade.** Choosing evaporative cooling improves the figure and consumes water; choosing dry cooling consumes electricity and worsens it. A better figure can mean a worse local outcome; see [[wiki/Data Center Water Use|Data Center Water Use]]. - **Useful work.** The denominator counts power delivered to servers, including servers that are idle. A facility full of underused machines can show an excellent ratio; see [[wiki/Utilization|Utilization]]. - **Load fraction.** Efficiency varies with how loaded the plant is, so a figure measured at design load differs from one measured annually; honest reporting is annualized. - **Boundary choices.** Where the measurement is taken and what is counted admit enough variation that comparisons across operators deserve care. - **Server fans.** Fans inside a server are counted in the denominator as computing load, which means moving cooling into the chassis improves the ratio without improving the facility. Liquid cooling, which removes those fans, can therefore make the figure look slightly worse while making the building genuinely better. ## The companion metrics Water usage effectiveness reports water consumed per unit of computing energy; carbon usage effectiveness reports emissions per unit; and measures of delivered work per unit of energy, such as tokens or training throughput per megawatt-hour, come closest to the question anyone actually cares about. The useful practice is to report the set rather than one figure, because each one discloses what the others conceal. ## Stack position - **Upstream:** climate, design temperature, cooling and electrical architecture. - **Downstream:** the operating cost of every unit of compute, and the facility's public environmental account. - **Function and vendor:** a measurement rather than a technology; its value lies in being measured consistently over time at one site. - **Degraded case:** optimizing the ratio alone can push a design toward water consumption and away from honest accounting of useful work. ## Relationships - **measures:** [[wiki/Data Center Cooling|Data Center Cooling]] · [[wiki/Chilled Water Plant|Chilled Water Plant]] · [[wiki/Uninterruptible Power Supply|Uninterruptible Power Supply]] - **traded against:** [[wiki/Data Center Water Use|Data Center Water Use]] · [[wiki/Cooling Tower|Cooling Tower]] - **complemented by:** [[wiki/Utilization|Utilization]] · [[wiki/Heat Reuse|Heat Reuse]] - **source articles:** [[articles/Vertically Integrating an AI Superpower from AI Factory to Citizen|Vertically Integrating an AI Superpower from AI Factory to Citizen]] · [[articles/The Gigawatt Is the New Measure of War|The Gigawatt Is the New Measure of War]] - **collection route:** [[collections/American Infrastructure|American Infrastructure]] · [[collections/American|American Collection]] ## Evidence posture - **Established:** the metric's definition and standardized measurement practice; the industry-wide improvement from figures commonly above 2.0 toward the region of 1.2 and better in well-designed facilities; the inclusion of server fans in the computing denominator; the dependence of the figure on climate, load fraction, and measurement boundary; the water-for-electricity trade. - **Strongly indicated:** work-per-energy metrics displacing the ratio as the headline figure; reporting practice is moving that way and no single standard has consolidated. ## Sources - International standards and industry practice for data center energy-efficiency metrics, as current October 2026.