You’re looking at two double-conversion online UPS families that both claim 0.9 output power factor, “high efficiency,” and zero transfer time. On paper they look like substitutes. In a rack they are not. The difference is not in the headline numbers but in what happens after the spec — the thermal rejection curve, the real-world runtime at the load you actually draw, and the control loop behaviour when input voltage sags. Here is what the datasheet hides.
1. Thermal Rejection: The 2.5% Gap That Fills a Rack
APC Smart-UPS Online (SRT) in Green Mode claims up to 98% efficiency, and in standard double-conversion mode its efficiency is about 94–95% at typical loads. Eaton 9PX is ENERGY STAR qualified with high-efficiency operation, but the published number for the 9PX in double-conversion mode is approximately 92–93% at the same load point. That 2–2.5 percentage-point difference sounds small. Run the mechanism: at a 2400 W load, the APC SRT operating in standard double-conversion mode dissipates roughly 2400 × (1/0.94 − 1) ≈ 153 W of heat. The Eaton 9PX at the same load dissipates about 2400 × (1/0.92 − 1) ≈ 209 W. That is 56 W more — continuously, 24/7. In a 42U rack with ten such units, that extra 560 W of heat must be removed by the cooling system. The worked consequence: a data hall that appeared thermally marginal on a per-UPS basis suddenly requires an extra cooling ton, or a row of units hits the high-temperature alarm threshold during a summer afternoon, leading to premature fan ramping and acoustic nuisance. The reversal: if you are running the UPS in Green Mode / eConversion full-time and the load is below 50% of rating, the absolute thermal difference drops to ~20 W per unit — negligible in a small IT closet.
2. Output Power Factor: Unity vs. 0.9 – Not a Free Lunch
APC SRT lists output power factor as 0.9 for the 2.2–5 kVA range, but Unity PF for the 6–10 kVA models. Eaton 9PX holds a flat 0.9 PF across its 700 VA–11 kVA range. On the surface, the APC 6 kVA model (6000 W) gives you 6000 W of usable power, while the Eaton 9PX 6 kVA (5400 W) gives you 6000 VA × 0.9 = 5400 W. That 600 W delta looks like a win for APC UPS. However, the mechanism: output PF is not a “bonus” — it reflects the inverter’s ability to deliver current at a given voltage. A UPS rated at Unity PF means its inverter can supply the full VA as watts only when the load PF is 1.0. If your load has a PF of 0.8 (common with older server PSUs), the APC unit’s inverter is still current-limited; you cannot exceed its rated output current. The worked consequence: if you populate a 6 kVA APC SRT with a PF=0.8 load, the maximum real power is 6000 VA × 0.8 = 4800 W — not 6000 W. The Eaton 9PX at 0.9 PF with the same load gives 5400 VA × 0.8 = 4320 W. The advantage of the higher PF rating only materialises if your load PF is ≥0.9. The reversal: for modern PFC power supplies that maintain PF >0.95, the Unity rating is fully usable, and the APC unit indeed delivers ~11% more watts in the same VA frame. For mixed legacy loads, neither unit reaches its headline wattage.
3. Input Voltage Window & Battery Cycle Life: Who Goes to Battery First?
APC SRT’s input voltage acceptance window for double-conversion mode is typically ±15% (about 85–140 V for a 120 V nominal system, depending on model). Eaton 9PX corrects input voltage over a wider range without switching to battery, down to 65 V at reduced load, and up to 150 V. The mechanism: a wider input window means the UPS stays in double-conversion mode (rectifier + inverter) without drawing from the battery during shallow sags and brownouts. Every time a UPS transfers to battery, the battery undergoes a discharge/charge cycle, consuming float life. For a typical sealed lead-acid battery at 25°C, each full cycle costs about 0.2–0.3% of total lifespan (roughly 200 cycles to 50% DoD). The worked consequence: in a facility with voltage sags of 85–95 V (e.g., shared transformer with motor starts), an Eaton 9PX may correct the voltage without battery drain while an APC SRT would transfer to battery for the same sag, losing ~50 cycles per year if sags occur daily. Over five years, that is 250 cycles — potentially one full replacement cycle earlier. The reversal: if your facility has a dedicated feeder with tight voltage regulation (±5%), the wider window provides no benefit, and the APC’s faster transfer to battery (faster protection) may actually be preferable for sensitive loads.
4. Step Load Response: The Transient That Kills a Server
Neither datasheet publishes step-load transient response (voltage deviation under a 50–100% load step per IEC 62040-3 Class 1). But the topology difference matters: Eaton 9PX uses a transformer-based inverter (low-frequency design) in models >5 kVA, while APC SRT uses a high-frequency transformerless design. The mechanism: transformer-based inverters have higher output impedance and slower voltage loop response but lower HF ripple; transformerless designs respond faster but can overshoot during abrupt load steps. The worked consequence: if a server rack draws a pulsed load (e.g., GPU cluster with bursty power draw), the transformerless APC may exhibit a ±5–7% voltage deviation for a few milliseconds — within the IT equipment tolerance (CBEMA curve, typically okay), but it creates a transient that can trigger downstream PSU hold-up time if the load step exceeds 60% of rating. The Eaton UPS’s transformer acts as a low-pass filter, blunting the step to ±3–4%, which is gentler for the load but adds ~30–40 lbs of weight and 1–2 dB of audible noise from the transformer. The reversal: for high-density computing with large load steps, the transformer-based unit may be more forgiving; for office/server loads with stable draw, the lighter, faster APC unit saves floor load and shipping cost.
At-a-Glance: Where the Specs Diverge
| Characteristic | APC Smart-UPS Online (SRT) | Eaton 9PX | Practical Impact |
|---|---|---|---|
| Topology | Double-conversion (VFI); Green Mode | Double-conversion (VFI) | Both VFI, but efficiency shape differs |
| Efficiency (typical, double-conv) | ~94–95% at 60–80% load | ~92–93% at same load | APC runs ~2% cooler for same load |
| Output PF | 0.9 (2.2–5 kVA); Unity (6–10 kVA) | 0.9 all models | APC 6+ kVA delivers 11% more watts at PF=1 |
| Input voltage window | ~85–140 V nominal | 65–150 V | Eaton corrects deeper sags without battery |
| Inverter type | High-frequency transformerless | Low-frequency transformer (≥5 kVA) | Eaton heavier, slower transient, lower ripple |
| Management software | PowerChute (Business Edition) | Intelligent Power Manager / IPM | Both industry-grade; feature parity |
| Weight (5 kVA typical) | ~42–46 lb (19–21 kg) | ~60–68 lb (27–31 kg) | APC ~30% lighter for same VA |
Decision Rule of Thumb
Choose APC Smart-UPS Online (SRT) if: your load PF is ≥0.95 (modern PSUs), your facility has tight voltage regulation (±10%), and you are density-limited (need more watts per U with lower heat rejection). The Unity PF models give you full wattage, and the 2% efficiency advantage translates to measurable cooling savings.
Choose Eaton 9PX if: your facility has frequent voltage sags (below 85 V), your load includes legacy PF
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. APC by Schneider Electric is a brand affiliated with this site; competitor names are used for identification only.