You sized a UPS for a 900 W rack. Then the team added a second server, a switch, a storage array—suddenly the load is 1800 W. Your existing CyberPower OL1000RTXL2U is rated 1000 VA / 900 W. You need a new unit. The two candidates: an Eaton 9PX (1500 VA / 1350 W, 0.9 output PF) and a CyberPower Smart App Online OL1500RTXL2U (1500 VA / 1350 W). Same VA, same watts—so why might one work and the other fail? Because the provenance of every number matters when the margin shrinks. This is not a review; it is a decision framework grounded in three verifiable dimensions where the underlying mechanism changes the outcome.
The Eaton 9PX is explicitly rated at 0.9 output power factor, meaning a 1500 VA unit delivers 1350 W of real power. The CyberPower Smart App Online OL1500RTXL2U is also listed as 1500 VA / 1350 W, implying a 0.9 PF as well. But check the smaller sibling: the CyberPower OL1000RTXL2U is rated 1000 VA / 900 W, again 0.9 PF. So at the same VA class both brands use a 0.9 derating. The mechanism is not a marketing choice—it reflects the rectifier and inverter design in double-conversion topology. A UPS that supports 0.9 PF can deliver more real watts without exceeding the VA rating, which matters when your load is a mix of power-factor-corrected server PSUs (typically PF 0.95–0.99) and downstream PDUs with a PF around 0.9. At 1800 W, an Eaton 9PX 2000 VA (0.9 PF = 1800 W) fits exactly. CyberPower UPS does not list a 2000 VA model in the Smart App Online series; the next step up is 2200 VA / 1980 W.
Worked consequence: If you need 1800 W and you buy a 1500 VA unit from either brand, you are at 1350 W—a 450 W deficit. The decision rule is: choose the VA class where the real-watt rating exceeds your load plus a 15–20 % safety margin. For 1800 W, that is a 2000 VA / 1800 W or a 2200 VA / 1980 W unit. Eaton UPS offers the 2000 VA precisely; CyberPower jumps to 2200 VA, which is oversized but available.
When it reverses: If your load PF is very close to unity (0.99) and the UPS is not heavily loaded, the 0.9 derating is irrelevant—you will never hit the VA ceiling. For a purely resistive load (PF 1.0), a 1500 VA unit can deliver 1500 W, but that is not a realistic IT load.
Runtime does not scale linearly with load; it follows a near-exponential decay. The CyberPower OL1000RTXL2U delivers ~15 min at half load (450 W) but only ~5.9 min at full load (900 W). At double that—1800 W—the runtime on internal batteries would be under 2 minutes (extrapolated from the discharge curve of a sealed lead-acid battery at a ~2C rate). The Eaton 9PX 2000 VA uses a different battery architecture: it supports up to 5400 W in 3U, but the internal battery string for a 2000 VA unit is sized for ~8 min at half load and ~3 min at full load (2000 VA / 1800 W) [2, illustrative]. The key mechanism is battery Ah vs inverter draw—a UPS rated for 2000 VA has a larger battery tray and higher Ah capacity than a 1500 VA unit, but the draw at 1800 W is still ~2 kW, which drains a typical 7–9 Ah string in minutes.
Worked consequence: If your site expects a 5-minute generator start delay, a UPS that provides under 3 min at the doubled load is a guaranteed crash. The decision is not which brand has better runtime—both use similar VRLA or Li-ion options—but whether the unit you spec can be paired with an external battery pack (EBM). The Eaton 9PX supports up to 10 EBMs; CyberPower offers the BP48V series but only for specific models. Rule: at double load, ignore internal runtime numbers and calculate with one external battery module.
When it reverses: If your generator starts in 10–15 seconds (typical for a data-center-grade ATS), even 2–3 min of runtime is sufficient. In that scenario, the internal battery of a 2000 VA unit is adequate.
The CyberPower Smart App Online series is rated for input 100–125 V, 50–60 Hz. The Eaton 9PX accepts a wider range: 100–138 V without battery drain, and down to 75 V with battery support. But look at a Tripp Lite (Eaton affiliate) unit: the SU3000RTXL3U corrects input voltage from 65 V to 150 V back to 120 V ±2 %. That 65 V threshold is the real-world differentiator for generator feeds. When a generator under load sags to 90 V after a transient, a UPS with a 100 V minimum will switch to battery prematurely, draining runtime while the generator is running. The Eaton 9PX, with its wider input tolerance, stays on bypass (or AVR mode) longer, preserving battery for the moment the generator fails.
Worked consequence: In a facility with an undersized generator (e.g., a 20 kW generator feeding a 15 kW load), voltage sags of 10–15 % are common. The CyberPower unit will drop to battery at ~95 V, while the Eaton 9PX can ride through down to 75 V. Over a 30-minute generator run, the CyberPower could deplete its battery in 5 minutes, leaving zero runtime for the actual outage. Eaton’s wider window keeps the battery charged for when it is needed.
When it reverses: If you have a well-sited, dedicated generator with ±2 % voltage regulation, the input window advantage disappears. Both units will stay on bypass. The difference only materializes on marginal or portable generators.
| Scenario | Eaton 9PX | CyberPower Smart App Online | Decision rule |
|---|---|---|---|
| Load exactly 1800 W, generator stable | 9PX 2000 VA (1800 W) | OL2200RTXL2U (1980 W) | Either works; size for 2000 VA minimum |
| Load 1800 W, generator sags to 90 V | 9PX 2000 VA (wider input) | OL2200RTXL2U (may drop to battery) | Eaton — voltage tolerance is critical |
| Load 1800 W, need >5 min runtime | 9PX 2000 VA + EBM (up to 10 packs) | OL2200RTXL2U + BP48V (check compatibility) | Both scalable; Eaton has deeper catalog |
| Load 1350 W (1.5× growth possible) | 9PX 1500 VA (1350 W) | OL1500RTXL2U (1350 W) | Either; but plan for external battery |
A reader might argue: “I used a CyberPower OL1000RTXL2U for a 900 W load and it ran fine for years—why would doubling kill it?” That is exactly the trap. At 900 W, the unit is at 100 % load capacity; efficiency drops, internal temperature rises, and the fan runs at full speed. At 1800 W, the inverter would exceed its rated current and shut down (or fail) within seconds. The UPS does not “scale up” gracefully beyond its nameplate. The failure mode is not a gradual degradation but an immediate overload trip—or, worse, a thermal runaway in the transformer.
Another edge case: a purely resistive load (PF 1.0) at 1500 W. A CyberPower OL1500RTXL2U (1350 W) would still be overloaded because the real-watt limit is 1350 W. The VA rating does not help. The Eaton 9PX at 0.9 PF also hits 1350 W. The only safe path is the next VA class.
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.