Topology is the first specification decision on any uninterruptible power supply, and it sets the ceiling on both protection and efficiency for the life of the machine. Every model in the SecondWatt UPS systems catalog is described first by its topology and then by its rating, because those two facts govern almost everything else on the datasheet. What follows is what the topology classes mean, what each one protects against, and how to read the efficiency numbers manufacturers publish against them.
This guide is one of four spokes beneath the data center UPS and energy storage hub, which sets out how topology, storage, sizing and frame architecture fit together. Read it alongside UPS energy storage compared and modular vs monolithic frame architecture.
Key Takeaways
- Topology naming is a standard. IEC 62040-3 assigns every UPS a three-part code whose first part is VFI, VI or VFD, and a vendor claim without that code is incomplete.
- Double conversion (VFI) decouples the load from both voltage and frequency, and is the topology used by almost all three-phase data center machines.
- Silicon carbide has narrowed the efficiency penalty: 98% in true double conversion on the Mitsubishi SUMMIT Series, 97.5% on the Vertiv Liebert APM2.
- The 99%-plus figures in the market are eco mode, where the unit runs on static bypass and is behaving as VFD. That is a protection-class trade, priced in points of efficiency.
- Ask three questions of any efficiency claim: at what load point, in which mode, and against which IEC classification code.
The classification is a standard#
Topology naming is governed by IEC 62040-3, the international standard specifying performance and test requirements for UPS. The standard assigns each unit a three-part code. The first part is the input dependency class: VFI (voltage and frequency independent), VI (voltage independent) or VFD (voltage and frequency dependent).
The remaining characters describe output waveform and dynamic performance during load steps and transfers. Most vendor material quotes the first three letters and stops, which is where the useful detail lives.
Table 1 — IEC 62040-3 input dependency classes#
| Class | Output decoupled from | Utility disturbances passed to load | Typical application |
|---|---|---|---|
| VFI | Voltage and frequency | None in normal operation | Three-phase data center UPS |
| VI | Voltage only; frequency follows input | Frequency variation | Single-phase rack and tower units |
| VFD | Neither; unit intervenes on supply failure | Voltage and frequency variation | Small office equipment, and any unit running in eco mode |
VFI means the output is decoupled from both the voltage and the frequency of the incoming supply. VI means the output voltage is regulated while the frequency follows the input. VFD means the output tracks the input on both counts, and the unit intervenes only when the supply fails outright.
Double conversion (VFI)#
A double-conversion UPS rectifies incoming AC to DC, then inverts it back to AC. The load is permanently fed from the inverter, so the input supply never reaches it directly. Utility disturbances such as sags, harmonics and frequency drift are absorbed by the DC link.
This is the topology of almost all three-phase data center machines. The Mitsubishi Electric 9900AEGIS and the Toshiba G9000 are conventional examples in the 80 kVA to 2,000 kVA band. The cost is conversion loss: two power stages run continuously, and every watt lost becomes heat the cooling plant then has to remove.
Silicon carbide switching has narrowed that penalty considerably.
98% in true double conversion — the Mitsubishi SUMMIT Series holds it at 500 and 750 kVA, without giving up VFI protection to do so.
97.5% in double conversion — the Vertiv Liebert APM2 reaches it using a hybrid silicon carbide converter.
Those two figures matter precisely because they are earned in VFI. They are the honest comparison point for any eco-mode number quoted alongside them.
Line-interactive (VI)#
A line-interactive UPS regulates voltage with a tap-changing transformer or a bidirectional converter, and switches to battery only when the supply falls outside its window. It is cheaper to build and runs at higher efficiency, because no continuous double conversion is taking place. It is also common in the single-phase rack and tower band rather than in the data hall.
The trade-off is frequency. A VI unit follows input frequency, which is acceptable for most IT loads on a stable grid and unacceptable where a generator with a slow governor sits upstream. Small online units such as the Vertiv Liebert GXT5 exist partly because operators want VFI behavior at rack scale.
Eco mode is an operating mode#
Eco mode runs the load on the static bypass with the inverter idling, ready to pick up. Efficiency rises because the conversion stages are not carrying the load, and the published numbers are high: 99% for the Eaton Power Xpert 9395P in Energy Saver System, and 99.2% for the Vertiv Liebert EXM2. While the unit sits in that mode it is behaving as VFD.
That is the whole decision. You are trading a documented protection class for one to two points of efficiency, and the exposure lasts as long as the transfer takes.
Table 2 — Published efficiency by model and operating mode#
| Model | Class in the quoted mode | Published efficiency | Mode the figure describes |
|---|---|---|---|
| Vertiv Liebert EXM2 | VFD while in mode | 99.2% | Eco mode |
| Eaton Power Xpert 9395P | VFD while in mode | 99% | Energy Saver System |
| Mitsubishi SUMMIT Series | VFI | ~98% at 500 and 750 kVA | True double conversion |
| Vertiv Liebert APM2 | VFI | 97.5% | Double conversion, hybrid SiC |
Read that table top to bottom and the point is unmissable: the two highest numbers describe a different protection class from the two below them. A 99.2% claim and a 97.5% claim are not competing on the same terms.
Some operators run eco mode only outside peak load hours, some enable it only while upstream power quality is monitored, and some never enable it. Whichever position you take, record it as a deliberate setting so it does not sit at the factory default by accident.
Efficiency is also more than an energy bill. The US Department of Energy tracks data center and server energy use as a national load category, and the Lawrence Berkeley National Laboratory data center energy usage report remains the reference dataset behind most facility-level modeling. ENERGY STAR certifies UPS against a tested efficiency curve, which compares better across vendors than a single headline percentage. Where a purchase decision turns on those points of efficiency, the capital side belongs in the model too — current benchmarks by rating are in the data center UPS pricing guide.
Rotary and flywheel machines#
Not every UPS stores energy chemically. The Piller UNIBLOCK UBT+ is a mechanically coupled rotary UPS built from 150 kW to 50 MW, and the Active Power CleanSource HD is a battery-free flywheel machine rated 675 kW. The VYCON VDC is a DC-bus flywheel delivering 300 kW for about 16 seconds, and attaches to an existing static UPS.
Short ride-through is the design intent. Where the generator start sequence is proven and tested to NFPA 110, seconds of stored energy cover the transfer and the site avoids a battery room entirely. That design leans on generator availability, so check current generator lead times before committing to it.
Reading a topology claim#
Three checks separate a specification from a brochure.
Ask for the IEC 62040-3 classification code in full, not just the letters VFI. Ask at what load point the quoted efficiency was measured, because part-load efficiency is where most installed UPS actually operate. Ask whether the figure describes double conversion or eco mode, and confirm which mode the site will run in.
Power remains one of the most-reported causes of significant outages in the Uptime Institute annual outage analysis, and topology choice is where that risk is either priced in or engineered out. Cross-check the electrical installation requirements against NFPA 70 before the layout is fixed, since clearance and overcurrent decisions are difficult to revisit once the room is built. The same sequencing applies across the wider standby stack, where backup power lead times move independently of the UPS.
FAQ: UPS topology selection#
What is a double conversion UPS?#
A UPS that rectifies incoming AC to DC and inverts it back to AC, so the load is fed permanently from the inverter and never directly from the utility. It carries the VFI classification under IEC 62040-3, meaning the output is independent of both input voltage and input frequency. It is the standard topology for three-phase data center installations.
What is the difference between VFI, VI and VFD?#
VFI decouples the load from both voltage and frequency. VI regulates voltage while the output frequency follows the input. VFD does neither and intervenes only when the supply fails. The classes describe what the load is protected from during normal operation, which is why a unit's class can change when it switches operating mode.
Is eco mode safe to run?#
It is a deliberate reduction in protection class, and whether that is acceptable depends on upstream power quality and on what the load tolerates. While a machine runs in eco mode it is behaving as VFD, so utility disturbances reach the load until the inverter picks up. Sites that enable it typically do so under monitored conditions or outside peak hours, and record the setting explicitly.
What efficiency should a data center UPS achieve?#
In true double conversion, current silicon carbide machines reach roughly 97.5% to 98% at the ratings above. Figures of 99% and higher describe eco mode. Compare like with like: a vendor quoting 99.2% and a vendor quoting 97.5% may both be accurate and describing different protection classes.
Can you buy a used UPS and keep the same topology?#
Yes. Topology is a property of the machine, so a secondary-market unit carries the classification it was built with. The variable is the energy store, which ages on a calendar as well as a cycle count and is usually priced as a separate item.
Next step#
Filter by topology, rating and manufacturer in the UPS systems hub, where each model page carries the published classification, efficiency figures and lead-time data behind these comparisons.