In short. A UPS protects the instant: it bridges the gap between a supply failure and the generator, and cleans the supply while it exists. Design depends on load criticality, runtime, redundancy, battery type and the generator that follows.
Principle
UPS protects the instant; the generator protects the duration. Battery runtime is chosen to cover the generator start and transfer, plus a safety margin, not an entire outage.
Decision map
| Decision | See |
|---|---|
| Topology: online double conversion | Online UPS |
| Three-phase and industrial | Three-phase UPS |
| Scalability and redundancy | Modular UPS |
| Batteries | Batteries |
| Bypass | Bypass |
| Generator interface | UPS and DG |
Sizing
UPS kVA ≥ (load kW ÷ load power factor) with margin, and kW within the UPS kW rating
Check both kVA and kW ratings, and the efficiency: input power equals output power divided by efficiency, and the losses become heat in the room.
Battery mindset
Battery voltage is not battery capacity. A bank can show correct voltage and fail at load. Test under load and trend internal resistance.
Redundancy
Redundancy needs independent failure paths. Two UPS units sharing one battery bank or one input breaker are not truly redundant.
Illustrative example
A data room runs a modular online UPS sized for current load plus growth, ten minutes of battery to cover the generator start and transfer, and a maintenance bypass. The generator takes over after the start and the UPS recharges.
Sizing a UPS: a worked example
A server room has equipment with a nameplate total of 24 kW at a power factor near 0.9 and measured typical load of 15 kW. Growth over three years is expected to be 30 percent.
- Planning load: 15 kW × 1.3 ≈ 19.5 kW.
- Required kVA: 19.5 ÷ 0.9 ≈ 21.7 kVA, so a 30 kVA unit is a reasonable class, subject to the UPS’s kW rating.
- Battery runtime: select to cover generator start and transfer plus margin, using the maker’s discharge tables.
- Efficiency: at, say, 94 percent efficiency, input power is about 19.5 ÷ 0.94 ≈ 20.7 kW, plus battery charging, and losses of about 1.2 kW appear as heat.
The numbers are illustrative; use maker data.
Architecture options
| Option | Resilience | Cost and complexity |
|---|---|---|
| Single UPS | Basic; single point of failure | Lowest |
| N+1 parallel redundant | Survives one UPS failure | Moderate |
| 2N, two independent systems | Survives a full path failure | High |
Room and environment
- Cool, clean, dry room with space for service.
- Battery temperature controlled, because life falls with heat.
- Fire detection and safe battery handling.
- Floor loading and cable routes planned.
Operating practices
- Test transfer to battery periodically, with plans for the load.
- Record battery and UPS readings.
- Maintain firmware and settings records.
- Train staff on bypass procedures.
When a UPS is not enough
For outages longer than the battery, add a generator and test the combined system. A UPS with no generator is a delayed shutdown, not continuity.
The master engineering question
If the utility fails now, what carries the load for the next ten seconds, the next ten minutes and the next ten hours?
What to send us
Send load list and criticality. Send your application details.
Questions engineers ask
How much runtime?
Enough to cover generator start, transfer and a margin.
Online or line-interactive?
Online for critical loads; others by need and budget.
Do I need a bypass?
For maintenance and fault resilience, yes.
Is UPS enough alone?
Only for short outages; add a generator for longer ones.
Values are illustrative. Use manufacturer data.