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A data-centre UPS turns incoming AC into DC, keeps a battery charged on that DC bus, and turns the DC back into clean AC for the IT load. When the utility fails the battery carries the load with no break, long enough for the generator to start; when the UPS itself needs work, a bypass carries the load around it.
Eaton's UPS basics paper describes the chain: the input rectifier converts AC to DC and feeds the output inverter, isolating critical loads from raw utility power; if the AC input falls out of limits the rectifier shuts off and the inverter keeps going on battery; on a severe overload or a converter failure, the static switch moves the load to bypass.
It converts every watt twice and keeps the battery charged. The Galaxy VS manual puts the trade-off plainly: double-conversion mode permanently creates a perfect sine wave at the output, but this also uses more electricity. That cost buys isolation: sags, spikes and frequency drift on the utility stop at the rectifier, and the load only ever sees the inverter. It is also why makers offer efficiency modes that run the load on the bypass while the utility is clean; see types of UPS for how those differ.
Nothing visible at the load. The rectifier stops, the battery carries the DC bus, and the inverter keeps the output steady. The Galaxy VS manual describes it simply: the UPS transfers to battery operation and supports the load with conditioned power from the DC source. Eaton puts a typical UPS battery at five to 15 minutes of backup, extendable with external battery cabinets; how long the generator takes to come online, and what it must handle, is covered in UPS and generator compatibility. The battery bridges the gap; the generator carries the long outage.
| Mode | Load is supplied by | Battery available? |
|---|---|---|
| Double conversion (normal) | The inverter, with a perfect sine wave, at the cost of more electricity | Yes |
| Battery operation | The inverter, from the DC source | It is in use |
| Requested static bypass | The bypass source, by command; returns to double conversion on a fault | Yes, if the utility is interrupted |
| Forced static bypass | The bypass source, after a UPS command or inverter off | No |
| Maintenance bypass | The bypass source, unconditioned, around the UPS | No |
| ECO mode | The static bypass while power quality is in tolerance | Charged, used on transfer |
The table follows the operating modes in Schneider Electric's Galaxy VS manual; other makers name them differently, but the logic is the same.
A modern UPS draws close to its rated current at a power factor near unity. Vertiv's catalogue describes active input power factor correction and low input current distortion that keep the current drawn from upstream close to the nominal output current, eliminating the need to oversize generators. Older and other designs differ: Cummins lists UPS rectifiers among the data-centre loads that create harmonics, and their filters among the loads with a leading power factor, both of which matter when a generator carries the UPS.
Because the UPS itself will need service and can fail. The static bypass catches the load in milliseconds if the inverter is overloaded or a converter fails; the maintenance bypass lets the whole UPS be isolated for work while the load stays powered. In both, the load runs on unconditioned power, which is why bypass time is planned. Details in static and maintenance bypass.
By maintenance and by testing it under load. A load bank test proves the power train at known steps; battery capacity tests prove runtime. For service by brand, see UPS service for data centres; for replacement batteries for data-centre UPS, see the data-centre UPS battery page.
No. In a double-conversion UPS the inverter is already carrying the load; when the rectifier stops, the battery simply takes over the DC bus.
Eaton puts a typical UPS battery at five to 15 minutes; data-centre systems are sized to bridge the generator start with margin, and external battery cabinets extend it.
To move the load to the bypass source almost instantly if the inverter is overloaded or a converter fails, so the load stays powered.
No. In static or maintenance bypass the load runs on unconditioned power from the bypass source, and on some UPS the batteries are not available.
Sources, checked 2026-10-04:
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