The shelter's thermostatic fan cycles on at 35°C, but the airflow over the UPS intake is blocked by a cable tray. Ambient inside the enclosure: 32°C steady, spikes to 38°C after a grid dropout because the compressor lags. You have 5 minutes to decide which double-conversion unit goes in: an Eaton 9PX (3U, 5400 W max) or an APC Smart-UPS Online (SRT) (3U, 5 kVA with Unity PF). The loading is 3.4 kW on a 4 kVA fan-inverter loop. The real question isn't which has better efficiency—it's which fails first when the cooling gets tight.
Failure mode: thermal runaway Both units are double-conversion (VFI), so during shelter operation they run 100% of load through the rectifier-inverter leg. At 3.4 kW (85% of a 4 kVA frame), the Eaton 9PX operates in its high-efficiency zone; it is ENERGY STAR qualified at typical loads, with about 94–95% efficiency for the 5–11 kVA range. The APC SRT (1–10 kVA) quotes up to 98% in Green Mode, but Green Mode is a bypass state, not double-conversion. In true double-conversion mode (the only permissible setting for a shelter with generator riding and frequency regulation), the APC SRT’s efficiency is roughly 92–93% at similar partial load (derived from typical VFI curve; manufacturer states "up to 98% in Green Mode" without a double-conversion efficiency number).
Worked consequence: At 3.4 kW, the Eaton 9PX wastes about 5–6% as heat ≈ 170–204 W dissipated inside the shelter. The APC SRT in double-conversion wastes about 7–8% ≈ 238–272 W. That extra ~70 W may not sound like much, but in a sealed shelter with 0.5 m³/min ventilation, 70 W continuous can raise internal temperature by 3–4°C (roughly, assume 1°C per 20 W in a small enclosure). That pushes the ambient toward 38°C, where every UPS’s internal fan speeds up and MOSFET junction temperatures climb. The Eaton UPS runs cooler by ≈70 W less heat, which buys about 2–3°C margin before the shelter's thermal trip.
When this reverses: If the shelter has an oversized CRAC unit and the UPS is never above 30% load (
Failure mode: overload trip on inverter The Eaton 9PX 3U model delivers up to 5400 W with a 0.9 PF rating, meaning its output stage is designed for 0.7 lagging to 0.9 leading without derating. The APC SRT in the 2.2–5 kVA range is also rated 0.9 PF, but the 6–10 kVA models offer Unity PF. The shelter load is a VFD fan inverter (≈0.85 lagging PF) plus control transformer and some LED lighting — net PF ≈ 0.8. At 3.4 kW on a 4 kVA UPS, the apparent power is 4.25 kVA (3.4 / 0.8). For the APC SRT at 5 kVA with 0.9 PF, the unit's real-watt capacity is 4500 W, but the inverter must supply 4.25 kVA — within rating. However, the Eaton 9PX handles 0.8 PF natively because its 0.9 PF rating is a minimum warranty spec; it can supply 3.4 kW at 0.8 PF without crossing the kVA ceiling (4.25 kVA
Worked consequence: The difference emerges not at steady state but during a transient. When the VFD accelerates the fan, inrush current reaches 2× rated for 200 ms. The Eaton 9PX uses a digital PFC controller that can support 110% overload for 10 min and 125% for 1 min. The APC SRT in the 5 kVA frame supports 105% for 10 min and 130% for 1 min. Both trip at 150% instantly. The Eaton’s higher overload tolerance (110% vs 105% at the longer window) means it will not drop the load during a moderate pump start. In a tight shelter, a single nuisance trip means the fan stops, temperature spikes, and the second UPS (if any) takes a cold start — that's the failure mode.
When this reverses: If the load is purely resistive (PF=1.0) or has no motor start, the overload margin difference becomes academic. For a shelter that powers only server gear (PF ~0.95), both units behave identically.
Failure mode: battery thermal runaway The Eaton 9PX accepts 100–138 V without switching to battery; the APC SRT (2.2–5 kVA) has a nominal input of 120 V ±15% ≈ 102–138 V. Both are comparable on paper. But the failure mode here is post-outage recharge. After a 15-minute outage at 3.4 kW, a 5 kVA UPS with internal batteries (about 3.6 kWh typical) will be depleted to ~30%. Recharge rate: the Eaton 9PX recharges at ~1.5 A per battery string (typical for 3U internal), taking about 4 hours to 90%. The APC SRT's internal charger is similar (≈1.2 A). During recharging, the battery heat generation increases ~20% due to internal resistance. In a shelter already at 35°C, that extra heat pushes the battery temperature toward 40°C, where lead-acid gassing accelerates and valve-regulated cells lose capacity faster. The Eaton 9PX includes a temperature-compensated charging circuit (derives charge voltage from ambient); the APC SRT also has temperature compensation but only with an optional probe. In the tight shelter, the Eaton's standard temperature sensor keeps the float voltage lower when hot, reducing thermal runaway risk.
Worked consequence: After two consecutive outage/recharge cycles in a 35°C shelter, the APC UPS battery internal temperature could exceed 42°C, reducing cycle life by ~50% (every 8°C above 25°C halves life). The Eaton, with active temperature compensation, stays below 38°C. The practical outcome: the APC unit may require battery replacement after 1.5 years, versus 3+ years for the Eaton in the same thermal environment.
When this reverses: If the shelter has a dedicated battery cabinet with active cooling, or if the UPS is oversized so recharge current is lower (e.g., using 10 kVA frame at 3.4 kW), the thermal stress differential disappears.
| Dimension | Eaton 9PX (3U, 5.4 kW) | APC Smart-UPS Online SRT (3U, 5 kVA) |
|---|---|---|
| Topology | Double-conversion VFI | Double-conversion VFI (Green Mode bypass available) |
| Heat rejection at 3.4 kW (illustrative) | ~170–200 W (94–95% eff) | ~240–270 W (92–93% eff, derived) |
| Output PF rating | 0.9 min (supports 0.8 PF without derating) | 0.9 PF (2.2–5 kVA); Unity PF (6–10 kVA) |
| Overload tolerance (long window) | 110% for 10 min | 105% for 10 min |
| Battery charge temp. compensation | Standard, integrated | Optional probe |
| Max internal batteries (3U) | ~3.6 kWh (10–12 Ah) | ~3.6 kWh (10–12 Ah) |
🔍 Non-obvious insight: The Eaton’s lower heat rejection (≈70 W less) matters more from a reliability standpoint than any efficiency spec sheet shows — because that 70 W is not just "waste," it’s the marginal heat that lifts battery temperature into the accelerated-aging zone. In a tight shelter, the UPS that runs cooler by 2–3°C will have batteries lasting twice as long. The APC’s headline 98% efficiency is irrelevant unless you’re willing to operate in bypass (Green Mode), which defeats the purpose of double-conversion in a shelter with unstable utility.
⚠️ Failure mode reversal: If the shelter’s cooling system is so poor that ambient reaches 45°C, both units will thermally fold back within 30 minutes — here the difference is academic. In that case, you need a 3-phase UPS with derating to 0.8 (like Schneider Galaxy VS) or external battery cabinets. The Eaton 9PX and APC SRT both have maximum operating temperature of 40°C; beyond that, any comparison is moot.
📏 Rule-based closing: If your shelter can keep UPS intake below 33°C and load PF ≥ 0.9, pick either unit — cost or service network decides. But if the thermal margin is tight (intake >33°C, ventilation only choice that reduces the probability of a thermal runaway event within the first 2 years.
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.