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What is a UPS?
A UPS, or uninterruptible power supply, keeps a load energised when the mains fails, using stored battery energy until either the mains returns or a generator takes over. It also conditions incoming power, protecting equipment from sags, surges, and frequency variation. Autonomy is typically measured in minutes, not hours, because a UPS bridges to another source rather than replacing it.
Most UPS problems are specification problems. The unit is sized for today's load, in an unventilated room, with batteries nobody tests. Here is what the three topologies actually do.
The three topologies
Topology decides how much of the mains disturbance reaches your load and how long the transfer takes.
| Topology | Transfer time | Protection | Typical use |
|---|---|---|---|
| Online double conversion | Zero | Full isolation from mains | Data centers, medical, process |
| Line interactive | 2 to 10 ms | Voltage regulation | Offices, light IT, network racks |
| Offline standby | 5 to 20 ms | Outage only | Workstations, non critical |
Sizing in four numbers
You need the total load in kW, the load power factor, the required autonomy in minutes, and expected growth over the asset life. Apparent power in kVA equals kW divided by power factor. Size the UPS to run at roughly 60 to 80 percent of rated capacity at commissioning, which leaves headroom for growth and keeps the unit in its efficient band.
What kills UPS installations
Three things, in order. Ambient heat, which halves battery life for every 10 degrees Celsius above 20. Untested batteries, where a single failed cell takes down the whole string at the moment of need. And silent load creep, where the protected load doubles over five years without anyone revisiting the sizing.
Frequently asked
- How long should UPS autonomy be?
- With a generator on site, 5 to 10 minutes is usually enough to cover start and transfer. Without a generator, autonomy is set by how long a controlled shutdown takes, commonly 15 to 30 minutes.
- What is the difference between kW and kVA?
- kW is real power consumed by the load, kVA is apparent power the UPS must supply. kVA equals kW divided by power factor. Modern IT loads sit near unity power factor, so the two figures are close.
- Can a UPS replace a generator?
- Not for long outages. A UPS gives minutes of clean power. Anything longer needs a generator or a correctly sized battery energy storage system designed for hours of discharge.