How this calculator sizes a UPS
Sizing a UPS correctly means satisfying two constraints at once. The UPS must deliver enough apparent power (kVA) to source the current the loads draw, and enough real power (kW) to do the actual work. Vendors publish both ratings because the ratio between them — the UPS's internal power factor — varies. An older online UPS might be rated 10 kVA / 9 kW (PF 0.9). A modern unity-PF UPS is rated 10 kVA / 10 kW. The same load list will choose differently between the two.
This calculator sums VA across every load row (converting W to VA using each load's power factor), adds your growth allowance (industry default 25%), then takes the larger of the resulting kVA requirement and the kW-derived kVA requirement (real-power constraint). Whichever is larger drives the recommendation up to the next standard UPS size — typically 1, 2, 3, 5, 6, 8, 10, 15, 20, 30 kVA, etc. A motor start surge, if you have one, is then checked against that frame's overload envelope.
Why power factor matters
Modern servers with 80 PLUS power supplies have a near-unity active PFC of ~0.98. Older non-PFC IT gear runs at ~0.88. Magnetic-ballast fluorescent or residential LED drivers can be as low as 0.6. A bank of motors averages 0.8. The mix matters: if your load is all 0.98 servers, the UPS PF rating barely matters. If it's mostly motors and ballasts, picking a 1.0-PF UPS lets you shave one or two size classes off your quote.
Motor inrush is special
A direct-on-line motor start draws roughly 6–7× its rated VA for the first few hundred milliseconds to a few seconds. Soft-start cuts that to ~2×; a VFD limits it to ~2× as well. Adding that surge to the continuous rating — the way many quick calculators do — turns a 15 kVA quote into 60 kVA. Instead, this calculator computes the peak (steady load after growth plus the inrush excess) and checks it against the UPS's published short-term overload capacity: typically 125 % for 10 minutes or 150 % for 60 seconds on an online double-conversion UPS, 200 % for a few seconds on some transformer-based units. The frame only steps up when the peak exceeds that envelope. What the arithmetic cannot know is the overload duration the vendor guarantees — confirm it covers the motor's actual start time, which for a large DOL start can exceed 10 seconds.
Topology recommendation
For loads under 3 kVA where the connected gear isn't especially mains-sensitive (consumer desktops, small comms cabinets, SoHo servers), a line-interactive topology is fine and cheaper. Between 3 and 10 kVA, online double-conversion becomes the default — it's what data-centre and clinical environments expect. Above 10 kVA, online double-conversion is effectively mandatory for the load types that justify the size in the first place. The calculator follows this convention but flags the choice so you can override it.
Standards behind the math
Growth allowance: Eaton/Tripp-Lite baseline 15% over 5 years; Riello recommends 20-25%. Power factor mapping: typical OEM datasheet values from Eaton 9PX, APC Smart-UPS, Schneider Galaxy. UPS topology classification follows IEC 62040-3. None of the public sources gives an exact multiplier for “UPS PF vs load PF” — the kVA-vs-kW dual constraint check used here is the standard cross-vendor approach.