Guides
A modular UPS is built from power modules in a shared frame, added or swapped as the load changes; a monolithic UPS is one large power unit, scaled by paralleling whole units. Both are double conversion; they differ in how they grow, how they are serviced and where their single points of failure sit.
A modular UPS is a frame holding several power modules, each with its own rectifier and inverter, sharing a bypass, controls and output. Capacity grows by adding modules: Tripp Lite's SVX uses 30 kW modules up to 210 kW in one frame, and XPC's M90U scales with 15 kW or 20 kW modules. Redundancy comes from one module more than the load needs, and service from swapping a module while the others carry the load; ABB describes its DPA modules as operating completely independently of each other and swappable online.
A monolithic UPS is one power unit sized for the load, with its own rectifier, inverter and static switch. It scales by paralleling whole units: Toshiba's G9000 supports parallel systems of up to 4 units in the smaller frames and 8 in the larger ones. Large monolithic UPS, such as Vertiv's Liebert EXL S1 at 100 to 1200 kVA, are common in large data centres, where whole units in parallel or in 2N give both capacity and redundancy.
| Modular | Monolithic | |
|---|---|---|
| Growth | Add modules in the frame, up to its rating | Add whole units in parallel |
| Redundancy | N+1 modules inside one frame | Parallel units, or 2N systems |
| Service | Swap a module with the rest carrying the load | Isolate a unit in a parallel system, or use bypass |
| Shared parts | Frame, bypass, controls, output | Each unit has its own; the parallel bus is shared |
| Right-sizing | Install what the load needs now | Size each unit for the expected load |
It depends on how the load will grow and how the site is built. Modular suits loads that grow in steps, sites that want N+1 in one footprint, and teams that value fast module swaps. Monolithic suits large, stable loads and designs that already duplicate whole systems for 2N. Eaton's advice holds for both: estimate the needs for the next three to five years before choosing. Some product lines blur the line: Vertiv's Trinergy Cube is a large modular system from 150 kW to 3400 kW.
In a modular UPS, the shared parts: the frame's bypass, controls and output. Redundant modules protect against a module failing, not against those, so ask the maker which parts of the frame are shared, how the shared bypass is rated, and whether the controls are duplicated. In a parallel monolithic system, the parallel bus and output switchgear. In both cases a 2N design with two independent paths removes the shared point; see N, N+1 and 2N.
Modular systems move the routine work to module level: a failed module is swapped, often with the load still on inverter. Monolithic systems need bypass or a parallel partner for most internal work. Either way the batteries, fans, capacitors and connections need the same scheduled care; see UPS preventive maintenance. For model-level data on modular and monolithic three-phase families, see the three-phase UPS modular vs monolithic reference; for service on either, UPS service for data centres.
Not by itself. A modular UPS with a spare module survives a module failure; a monolithic system needs a parallel unit or a second system for the same. Shared parts set the limit in both.
On many modular UPS, yes: makers such as ABB and XPC describe online or hot-swappable modules, with the remaining modules carrying the load.
Not always. It avoids buying capacity before it is needed, but the frame is sized for the maximum, and per-kW costs differ by model.
Yes, by adding units in parallel where the model supports it, such as Toshiba's G9000 with up to 4 or 8 units.
Sources, checked 2026-10-04:
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