# Uninterruptible Power Supply Grid power is excellent and occasionally interrupted. A generator needs seconds to start and accept load. An uninterruptible power supply covers that gap, holding the facility up on stored energy long enough for the generators to take over, and conditioning the power it passes so that sags, swells, and transients never reach the equipment. > [!note] The one-sentence version > It buys the seconds between the lights going out and the engines coming up, and it filters the power the rest of the time. ## Why it exists Computing equipment tolerates very little interruption; a few cycles of lost voltage will drop a server. Grid disturbances lasting a fraction of a second are routine and often invisible to a household. Between the standards of the grid and the standards of a processor sits a gap that stored energy fills. The system also solves a second problem: it isolates the load from power quality events, so a utility fault clearing somewhere on the network does not propagate into a data hall as a voltage dip. ## Prerequisites - **Rectifier, inverter, and static bypass:** the three elements of the classic design; the rectifier charges the store and feeds the inverter, the inverter synthesizes clean output, and the static bypass transfers the load to raw utility power in a fraction of a cycle if the inverter fails. - **Double conversion, line interactive, and eco mode:** double conversion processes all power through rectifier and inverter, giving the best isolation at a small efficiency cost; line interactive and eco modes pass utility power through when it is healthy and switch to conversion when it degrades, recovering that efficiency. - **Energy store:** batteries, flywheels, or capacitors; see [[wiki/Data Center Batteries|Data Center Batteries]]. - **Ride-through time:** how long the store can carry the load, typically specified in minutes and sized to the generator start sequence plus margin. - **Distributed redundancy:** several modules sharing the load so that any one can fail or be serviced while the rest carry the facility. ## How it works Utility power enters the rectifier, which charges the energy store and supplies the inverter. The inverter continuously synthesizes the output waveform, so the load is fed from the inverter at all times and never sees the utility directly. When utility power fails, the rectifier stops contributing and the store discharges into the inverter; the output waveform does not change and the load notices nothing. Generators start, the [[wiki/Automatic Transfer Switch|transfer switch]] moves the facility to generator power, the rectifier resumes, and the store recharges. On return of utility power the sequence reverses, deliberately and under control rather than instantly. Redundancy is expressed in a standard notation. N is exactly enough capacity; N+1 adds one spare module; 2N is two complete independent systems. Dense facilities increasingly pair a modest central system with energy storage distributed into the racks themselves, which shortens the path between the store and the load; see [[wiki/High-Voltage Direct Current Distribution|High-Voltage Direct Current Distribution]]. ## Stack position - **Upstream:** [[wiki/Medium-Voltage Distribution|medium-voltage distribution]]. - **Downstream:** [[wiki/Rack Power Distribution Unit|rack power distribution]] and the servers; cooling pumps and controls are also commonly supported, because a liquid-cooled hall loses thermal margin within seconds of the pumps stopping. - **Function and vendor:** the function is essential wherever the workload cannot tolerate interruption; many qualified suppliers exist, and topology is a design choice rather than a vendor one. - **Degraded case:** every utility disturbance becomes an outage, and a facility experiences several a year. ## Lineage and trajectory Early installations used motor-generator sets and flywheels; static designs with rectifiers and inverters took over as power electronics improved, and the modular cabinet replaced the monolithic machine. The present direction is efficiency and placement: eco modes that cut conversion losses while retaining fast transfer, lithium chemistries that shrink the footprint and lengthen service life, and storage moving into the rack so that each rack rides through on its own. A further development is treating the store as a grid asset, so that the stored energy earns value by supporting the utility rather than only waiting for a fault; see [[wiki/Demand Response|Demand Response]]. ## Relationships - **fed by:** [[wiki/Medium-Voltage Distribution|Medium-Voltage Distribution]] - **stores energy in:** [[wiki/Data Center Batteries|Data Center Batteries]] - **hands over to:** [[wiki/Backup Generator|Backup Generator]] · [[wiki/Automatic Transfer Switch|Automatic Transfer Switch]] - **protects:** [[wiki/Rack Power Distribution Unit|Rack Power Distribution Unit]] · [[wiki/Data Center Cooling|Data Center Cooling]] - **configured for:** [[wiki/Redundant Power Feeds|Redundant Power Feeds]] - **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:** rectifier, inverter, and static bypass architecture; double conversion, line interactive, and eco operating modes; the N, N+1, and 2N redundancy notation; ride-through sizing against generator start time; support of cooling pumps and controls alongside compute load. - **Established (datestamped):** as of October 2026, redundant uninterruptible power with independent A and B critical power and cooling paths is standard practice in operator disclosures across the industry. - **Strongly indicated:** rack-level energy storage displacing part of the central system in the densest designs; products are shipping and long-term operating data is still accumulating. ## Sources - Industry standards and manufacturer technical literature on uninterruptible power supply topologies and redundancy classes, as of October 2026.