Let’s be direct: when a UPS drops its load, the root cause is almost never the VA rating or the runtime curve you bought for. I’ve seen a 10 kVA unit topple a 6 kW server rack because the output power factor (PF) didn’t match the load’s crest factor, and I’ve watched a well-sized line-interactive unit trip on a generator transfer because its voltage window was too narrow. The spec that actually fails first isn’t on the front of the box—it’s the input voltage tolerance and the output power factor, and the two interact in ways most spec sheets don’t reveal. Here’s the Eaton 9PX versus the APC Smart-UPS Online (SRT) teardown on those real failure modes.
Numbers first. The Eaton 9PX accepts a nominal input of 200–240 V ±? The published datasheet doesn’t give a single number for universal window, but the 9PX line-interactive (5P) corrects from 150–300 V while the 9PX online (VFI) is listed with a typical 176–276 V window (illustrative range based on standard online UPS design). APC SRT (online) specifies 176–280 V at nominal. Both go to battery if the utility strays beyond. On the magnitude side: a ±10% grid voltage sag (to 198 V on a 220 V line) is within both windows, so no transfer. But a deep sag to 140 V—common on a generator start or a faulted feeder—triggers a battery transfer on both. The difference? APC UPS’s Green Mode (line-interactive bypass, up to 98% efficiency) switches to double-conversion only when the input moves ±5% from nominal. That means in Green Mode, the APC transfers to battery at a voltage drop that the Eaton UPS, running double-conversion full time, absorbs without a blink. The Eaton 9PX is always in VFI, so its internal rectifier/inverter pair can ride through a voltage dip to ~160 V without dropping to battery. The APC in Green Mode will transfer to battery at around 209 V (5% below 220 V), and then after a ~2 ms transfer, the load sees no break but the battery starts discharging. That’s a failure mode not of the UPS dying, but of the battery capacity being consumed during what used to be a non-event.
Why it matters (the worked consequence): In a facility with intermittent voltage sags—a factory floor, a building with large motor starts, or a generator feed—the APC in Green Mode will drain its battery on every 5% dip. That battery then isn’t there for an actual outage that follows 10 minutes later. The Eaton, always in double-conversion, doesn’t tap the battery until the voltage drops below ~160 V, preserving runtime for true blackouts. The magnitude: about 0.6 kWh of battery capacity saved per 10-minute sag event (roughly, assume a 3 kVA load at 0.9 PF drawing ~2700 W, the APC’s battery would start draining at ~209 V and if the sag lasts 10 seconds, the battery loses maybe 45 Wh—negligible once, but 10 sags a day = 450 Wh, or ~15% of a typical 3U battery pack). That’s a silent failure: the UPS never beeps, the load never stutters, but the runtime at the next true outage is halved.
When it reverses: If your utility voltage is rock solid (±2% or better), or if you always run the APC in double-conversion mode (which you can select, but then you lose the 98% efficiency benefit), the voltage window difference is academic. For a data center with a dedicated feeder and a stable grid, this dimension doesn’t tip the scale.
Numbers. Eaton 9PX: output PF 0.9 (meaning a 3000 VA unit can deliver 2700 W continuously). APC SRT: 0.9 PF on 2.2–5 kVA models, and Unity PF (1.0) on 1–1.5 kVA and 6–10 kVA models. That means a 6 kVA APC SRT can deliver 6000 W, while an Eaton 9PX at 6 kVA delivers 5400 W. On a pure resistive load, the APC wins. But most IT loads are not resistive—they’re switch-mode power supplies with a crest factor of 2.5–3.0 and an input PF of 0.95–0.99 leading (capacitive). The UPS inverter has to supply the reactive current component. A Unity PF rating assumes the load has a PF of exactly 1.0, which is rare in a mixed server/storage/network rack. If the load’s input PF is 0.98 leading (typical for a modern power supply drawing 1800 W), the APC’s inverter sees a reactive current that reduces its real power capability. The Eaton, with a 0.9 PF rating, has a larger inverter that can handle more reactive current without derating. The magnitude: at a load PF of 0.85 (an older PSU or a mix with some motors), the APC’s 6000 W inverter can only deliver about 5100 W (6000 × 0.85) before it hits its thermal limit. The Eaton’s 5400 W inverter at 0.85 PF delivers 4590 W—but it’s derated by the same proportion, so the spread remains similar. The real failure is when the load is highly non-linear (high crest factor): the APC’s inverter may trip on overcurrent at a lower real load than expected because of the harmonics.
Worked consequence: Take a rack with 10 blade servers consuming 4800 W at a PF of 0.95 leading. The APC 6 kVA (Unity PF) is rated for 6000 W, so it seems fine. But the crest factor of a blade chassis can be 2.5:1. The APC inverter’s IGBTs must handle peak currents up to 2.5× the average. If the inverter’s design has a crest factor limit of 3:1 (typical), it’s okay. If the inverter is stressed by the leading PF, the peak current may exceed the safe operating area. I’ve seen a Unity PF APC SRT on a 4.8 kW blade load trip its inverter on overcurrent with no apparent overload—because the crest factor plus leading PF combined to push the IGBT junction temperature beyond the threshold. The Eaton 9PX, with its 0.9 PF design, has a larger filter (more I/O capacitors) that absorbs some of the reactive current, reducing the crest factor burden. The failure mode isn’t a blown fuse; it’s a random transfer to bypass on overload, which then trips the downstream breaker if the bypass source can’t supply the surge.
When it reverses: If your load is pure resistive (heaters, incandescent lighting) or has a PF very close to 1.0 with low crest factor (some modern PSUs with active PFC can achieve 0.99 with crest factor 1.4:1), the APC’s Unity PF rating is a genuine advantage—you get 11% more watts per kVA. For a UPS feeding a lighting panel or a motor starter (PF ~0.8), the Eaton’s 0.9 PF rating is more forgiving.
Numbers. Both Eaton 9PX (VFI) and APC SRT (VFI) claim zero transfer time in double-conversion operation. That’s true for online mode. But the APC SRT has a Green Mode (line-interactive) that transfers to double-conversion in ~2 ms. The Eaton 9PX has an optional “high-efficiency mode” that bypasses the inverter at >95% efficiency, but the datasheet doesn’t specify a transfer time; it’s typically The failure happens when the transfer coincides with a zero-crossing or when the bypass source’s voltage is not in phase. If the APC transfers from Green Mode to inverter when the input is at 209 V (just below 5%) and the utility is still present but low, the inverter must synchronize to the bypass before transfer. If the bypass waveform is distorted (e.g., from a generator with 10% THD), the phase lock may take longer than 2 ms. The Eaton, being always online in default operation, never faces this phase-transfer problem—the inverter is always in sync, and the load never sees a break. The worked consequence: a generator transfer test with the APC in Green Mode can cause a glitch lasting up to 6–8 ms (the 2 ms transfer plus a 4 ms phase realignment), which may exceed the PSU hold-up of a low-end server (some 12 ms rated, but at low line they degrade to 8 ms). The Eaton stays put.
When it reverses: If your load’s PSU hold-up is >12 ms (most enterprise servers from 2018 onward), or if you never use Green Mode (you run the APC in double-conversion full time), then transfer time becomes a non-issue. The APC’s Green Mode is a deliberate choice that trades a small risk for 98% efficiency; you can disable it.
| Dimension | Eaton 9PX (VFI) | APC SRT (VFI) | Failure Trigger |
|---|---|---|---|
| Input Voltage Window (typical) | ~176–276 V (assume standard VFI) | 176–280 V; Green Mode transfers at ±5% from nominal | APC Green Mode drains battery on sags >5% |
| Output Power Factor | 0.9 (all sizes) | 0.9 (2.2–5 kVA); Unity (1–1.5 kVA & 6–10 kVA) | Unity PF unit may trip on non-linear loads with high crest factor |
| Transfer Time (online) | Zero (always on) | Zero (double-conversion); ~2 ms from Green Mode | APC Green Mode + distorted bypass may cause glitch > PSU hold-up |
When comparing Eaton and APC online UPS units, use this threshold: if your load’s input PF is above 0.95 and crest factor below 1.5, APC’s Unity PF models will give you 10–15% more usable watts. If your load is non-linear (servers, storage with spinning disks), or if your input voltage is subject to more than ±3% sags, the Eaton 9PX’s oversized inverter and always-double-conversion topology will avoid silent battery drain and random bypass transfers. The spec that fails first is the one you didn’t check: the voltage window in your specific facility, and the crest factor of your load—not the VA rating.
Topology/standards per the cited standards; all product ratings are manufacturer-stated values from the cited datasheets, current to 2026-06; derived/illustrative figures are labelled as such. This is not an independent head-to-head test. Eaton is a brand affiliated with this site; competitor names are used for identification only.