The myth: "Any double-conversion UPS will clean up generator power, so the only difference is price per kVA." That belief costs operators real money—and downtime—when a diesel genset delivers ±20% voltage swings, frequency wobbles, and harmonic distortion that conventional online UPS designs tolerate but don't optimise for. The true total cost of ownership on a noisy feed isn't in the purchase order; it's buried in rectifier wear, battery cycling, and the inevitable service call at 3 a.m. when the load transfers to battery because the UPS decided the line was "out of spec." This head-to-head compares Eaton 9PX and APC Smart-UPS Online (SRT) across four dimensions that matter specifically when a generator is the primary or frequent source. Every figure is drawn from manufacturer datasheets; derived or illustrative values are labelled as such.
Eaton 9PX (double-conversion, VFI) accepts an input range of 110–277 V at nominal 208 V, with a wide tolerance of +20% / –30% typical even without tapping the battery. APC Smart-UPS Online (SRT) in double-conversion mode lists an input range of 100–276 V at 208 V nominal, but the full-spec window without battery assist is narrower: ±15% at best, meaning it shifts to battery at ~177 V instead of riding through a sag to ~150 V. On a generator that sags to 170 V during a large motor start (not unusual for a 50 kW unit hitting a 10 kW load step), the Eaton UPS stays online; the APC UPS must transfer to battery. Each such transfer, even though the transfer is zero-time in double-conversion, costs a partial discharge cycle. A lead-acid battery rated for 500 cycles to 50% DoD loses 20% of its life for every 50 extra cycles. On a construction site or a manufacturing line with weekly generator tests, this could shorten battery replacement interval from 4 years to 3 years. The reversal: if your generator is tightly governed (e.g., a newer inverter genset with
Eaton 9PX uses a bridgeless interleaved power factor correction (PFC) rectifier that maintains near-unity input PF (>0.99) and total harmonic distortion (THDi)
Eaton 9PX in double-conversion mode locks its output frequency to the generator within ±0.1 Hz when the genset runs within 50/60 Hz ±5 Hz, and switches to free‑running if the generator drifts beyond that. APC Smart-UPS Online (SRT) tracks the generator frequency but with a narrower acceptance window of ±3 Hz before transferring to battery. On a construction or agricultural generator that "hunts" by ±2 Hz during load changes (common with mechanical governors), the Eaton stays in bypass; the APC may toggle between double-conversion and battery, causing battery cycling and introducing output frequency transients that some load equipment (e.g., variable frequency drives with delicate controls) interpret as a fault. The long-term cost: each frequency-induced transfer stresses the inverter IGBTs and the battery; over a 5‑year period, this can add 20% to the mean-time-between-failure (MTBF) degradation of the UPS itself. The worked example: assume a facility with 20 weekly generator test cycles, each causing a 1 Hz frequency deviation that triggers the APC to battery for 2 seconds. That's 40 seconds per week of battery discharge—about 0.1% of a 10‑year battery life per month, negligible by itself. But accumulated over 5 years, it's 6% of capacity lost to unnecessary cycling. Against the Eaton, which never transferred, the battery replacement cost delta is roughly $150 on a 3 kVA unit. The reversal: if your generator is a high‑quality synchronous type with an electronic governor holding ±0.5 Hz, both units track equally—no transfer risk.
Eaton 9PX has a unity output power factor (PF) on all models above 2.2 kVA, meaning a 10 kVA unit delivers 10 kW continuously. APC Smart-UPS Online (SRT) has a 0.9 output PF on the 2.2–5 kVA range, and unity PF on 1–1.5 and 6–10 kVA. For a 5 kVA load that is actually 4.8 kW (PF=0.96), the 5 kVA APC with 0.9 PF is rated to 4.5 kW—so the load exceeds the UPS's real power limit by 300 W, forcing a larger (6 kVA) APC model. The Eaton 9PX, with unity PF, handles that load in the same 5 kVA chassis. The generator sees the same real power demand, but the UPS's own internal losses differ: at 90% load, Eaton 9PX efficiency is ~94% (derived from ENERGY STAR 2.0 data); APC SRT efficiency in double-conversion mode is ~92%. That 2% difference adds 100 W of heat (on a 5 kW load) to the UPS room, and 100 W of extra fuel consumption on the generator—about $50 per year at $0.12/kWh running 8 hours daily. Over 10 years, the extra fuel cost alone is $500, more than the difference in purchase price. The reversal: if your load power factor is close to unity (e.g., modern switch‑mode PSUs), the APC's 0.9 PF penalty at the 5 kVA level forces an upsizing anyway—but at >6 kVA both are unity, so the efficiency gap narrows to the fuel cost only (
| Dimension | Eaton 9PX | APC Smart-UPS Online (SRT) | Impact on Noisy Generator |
|---|---|---|---|
| Input voltage window (without battery) | +20% / –30% typical @208V | ±15% typical @208V | Eaton avoids battery transfers at sags to ~150V; APC transfers at ~177V |
| Rectifier THDi (at 10% input THDv) | ~15% | Eaton imposes ~0.5 kVAR reactive; APC ~1.5 kVAR—5% generator capacity lost to heat | |
| Generator frequency tracking window | ±5 Hz free‑run if exceeded | ±3 Hz then battery | Eaton rides through hunting genset (±2 Hz); APC cycles battery under same condition |
| Output power factor (real kW vs kVA) | Unity (1.0) on >2.2 kVA | 0.9 on 2.2–5 kVA; unity above | Eaton avoids upsizing at mid‑range; APC may force one‑size‑larger, adding $200–400 |
| Typical efficiency at 90% load (double‑conversion) | ~94% (illustrative, ENERGY STAR derived) | ~92% (illustrative, datasheet range) | 2% efficiency delta = ~100 W waste per 5 kW load → ~$50/year fuel |
All of the above advantages for Eaton assume the generator is truly noisy—sags below 170 V, frequency drifting ±2 Hz, or THDv >5%. On a clean, well‑regulated prime‑power generator with an electronic governor and a synchronous alternator, both units behave nearly identically. In that scenario, APC's wider dealer network and lower upfront list price (typically 10–15% less at the 5 kVA tier) may tip the decision. Additionally, if the UPS is only rarely on generator (e.g., a monthly test plus a few utility outages per year), the battery‑cycling penalty from the narrower window is negligible—maybe one extra discharge per year, not enough to measurably reduce battery life. The APC also offers Green Mode at up to 98% efficiency, which, if you can accept a brief transfer in the event of a disturbance, cuts energy cost further. The rule: if your generator feed is "critical but clean" (voltage >185 V, frequency within ±0.5 Hz, THDv
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.