Guides
Size a UPS by adding up the load it must carry, in kW and kVA, adding margin for growth and inrush, then allowing for redundancy and runtime. Check the result against the generator, and confirm it with the maker's sizing data before ordering.
From measurements if you have them, from nameplates if you do not. Eaton's basic procedure: list every device the UPS will protect, find the volts and amps each draws, multiply for VA, add them up, and multiply by 1.2 for growth. Measured load from the existing UPS or the distribution is better than nameplate totals, which usually overstate it. Eaton adds that faster growth calls for a multiple above 1.2, and that new servers tend to draw more than the ones they replace.
Both: the UPS must carry the load's real power (kW) and its apparent power (kVA). Older UPS were commonly rated at 0.9 output power factor, so a 100 kVA unit carried at most 90 kW; many current models are rated at unity. Vertiv notes that a unity power factor rating delivers more active power than a traditionally rated 0.9 power factor UPS, and XPC lists its M90U at unity, kW equal to kVA. Check the load in kW against the UPS's kW rating, not only kVA. For a model-by-model explanation, see the kW vs kVA reference for three-phase UPS.
The generator must carry the UPS input, including battery recharge, and hold voltage and frequency within the UPS's input window. Eaton's UPS basics paper suggests a generator of 1.5 to 3.0 times the UPS kVA rating, depending on the UPS design, alternator size, regulator type and fuel; its generator page gives 1.5 to 2 times the total load for UPS up to 25 kW, depending on fuel and governor. Cummins adds data-centre specifics, such as UPS rectifier harmonics and leading power factor from IT power supplies. Vertiv notes that a UPS with active input power factor correction avoids oversizing the generator. More in UPS and generator compatibility.
Sizing on kVA alone, then finding the kW is short; using nameplate totals and buying a UPS that runs at low load for years; or forgetting that redundancy only holds while the load fits in N. A UPS that runs well below its rating wastes efficiency, and one that runs close to it has no room for the next rack. Check the load at every maintenance visit.
Yes, because adding it later is harder. N+1 needs one module or unit beyond the load; 2N needs a second complete system and path. Eaton's advice to estimate the next three to five years applies to both capacity and redundancy: a system that is N+1 today can drift to N as the load grows into the spare.
We size replacements from the measured load, the growth plan and the redundancy design, and confirm with the maker's data before anything is ordered; see replacement planning. For installation and commissioning after sizing, see UPS installation and service.
Eaton's basic method multiplies the summed load by 1.2 for growth, more if fast growth is expected; plan for the next three to five years.
Check both. Many current UPS are rated at unity power factor (kW equals kVA); older ones at 0.9. The load's kW must fit the UPS's kW rating.
Not usually: it needs more battery. The UPS's charger and battery cabinets must suit the larger battery.
Eaton suggests 1.5 to 3.0 times the UPS kVA, depending on the UPS design, alternator, regulator and fuel. Confirm with both makers.
Sources, checked 2026-10-04:
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