“I sized by VA — why did my server rack drop at 70% load?”

Numbers. APC Smart-UPS Online (SRT) in the 1–1.5 kVA range delivers unity output power factor (1.0 PF), meaning 1500 VA = 1500 W. That looks generous. Schneider Smart-UPS Online (SRT) at 2.2–5 kVA is rated 0.9 PF, so a 2200 VA unit delivers 1980 W. But the trap: many 1500 VA line-interactive units (e.g., APC SMT1500) are 0.7 PF, i.e., 1500 VA × 0.7 = 1050 W max.

Mechanism. A server power supply with active PFC typically draws a power factor >0.95. If the UPS output PF is lower than the load PF, the inverter’s current limit triggers earlier than the VA rating suggests. The inverter is current‑limited; at low PF it can deliver less real wattage. IEC 62040‑3 classifies online (VFI) vs line‑interactive (VI), but it doesn’t mandate output PF — that’s a manufacturer spec.

Worked consequence. For a 1350 W server (PF ~0.98, so ~1378 VA), an APC SMT1500 (max 1050 W) will overload. A Schneider Galaxy VS or Smart‑UPS Online (SRT) at 1500 VA / 1500 W holds it with margin. The decision: if you buy by VA, you must check the real‑watt column. One number alone is a trap.

Reversal. This only applies when the load PF is near unity. If you have legacy equipment with PF ≤0.7 (some older switches, motors), a 0.7‑PF UPS is not penalized — you actually get the full VA. For pure resistive loads, both ratings converge.

Numbers. Schneider Galaxy VS (10–150 kW) is double‑conversion up to 97% at every load level, and eConversion high‑efficiency mode up to 99%, with no‑break transfer. APC Smart‑UPS Online (SRT) 6–10 kVA range offers unity PF and double‑conversion, with “Green Mode” up to 98%. Both topologies are VFI.

Mechanism. Efficiency matters most at partial load. A UPS running at 30% load with 94% efficiency wastes 6% of input as heat. eConversion mode bypasses the inverter when mains is clean, but keeps the inverter synchronised and ready — typical loss ~1% vs ~3–4% in double‑conversion. Over a year, a 9 kW load (10 kVA @ 0.9 PF) at 97% vs 94% efficiency means ~270 W vs ~540 W dissipated — roughly 240 kWh difference annually (illustrative, assumes 24/7).

Worked consequence. For an IT closet drawing 9 kW continuously, the Schneider Galaxy VS in eConversion saves ~120 W of heat that must be removed by cooling. That translates to lower total cost of ownership and potentially lower fan noise. The APC Green Mode also saves, but the stated maximum efficiency is 98% vs 99% for Schneider UPS.

Reversal. eConversion / Green Mode only works while mains is within voltage/frequency windows. If your site has frequent sags or noise, the UPS spends more time in double‑conversion, erasing the gain. For a facility with dirty power (e.g., industrial estate), the simpler double‑conversion with wide input window may be more reliable.

Numbers. Schneider Galaxy VS at 208 V covers 10–75 kW, with input power‑factor correction and harmonic filtering. APC Smart‑UPS Online (SRT) single‑phase range tops at 10 kVA; for three‑phase you move to other product families (e.g., Symmetra). For a 208 V 3‑phase load pulling 12 kW balanced (approx 33 A per leg at 0.9 PF), a 15 kVA UPS with 0.9 PF delivers 13.5 kW — just barely.

Mechanism. Three‑phase UPS sizing must account for per‑phase current, not just total kVA. A 15 kVA 3‑phase unit at 208 V supplies 41.6 A per leg. But if the load is unbalanced (e.g., 15 A on phase A, 30 A on phase B, 25 A on phase C), one leg hits current limit before total kVA is reached. The Galaxy VS’s PFC and harmonic filter help keep crest factor low, but the physical limit is the inverter IGBT rating.

Worked consequence. For a 12 kW, well‑balanced load, both platforms work. But if your rack PDUs have single‑phase 20 A branch circuits that create imbalance, you might need a larger UPS. The real‑watt per phase is the binding constraint — not the sum.

Reversal. If you can balance to within 10% per leg, the per‑phase advantage is moot. The Galaxy VS’s wide input voltage window (down to 65 V correction on many models) is more valuable for brownout protection than the raw power rating.