Modular UPS proposals often compress four different values into one word: capacity. A frame may accept 60 kW of power modules, contain only 45 kW today, provide 30 kW with N+1 module redundancy and support a battery runtime calculated at a different load. Each value can be correct. Treating them as interchangeable creates design, quotation and expansion errors.
GDF Technologies encountered the simplest version of the problem when a customer questioned an 18 kW proposal. The explanation was arithmetic: three installed 6 kW modules provided 18 kW. That answer resolved the immediate question, but a complete selection also has to state whether the figure is installed capacity, usable protected capacity or the limit of the frame.
Four capacities, four decisions
Frame capacity is the maximum power-module configuration supported by the architecture. Installed capacity is the sum of modules physically installed. Protected capacity is what can serve the load after any redundancy reserve is applied. Runtime is not power capacity at all. It is the battery-supported duration at a defined load, battery condition and endpoint.
Figure 1. The capacity terms answer different engineering questions. Graphic by GDF Technologies [1][2].
| Term | Question answered | Common error |
|---|---|---|
| Frame capacity | How much power hardware can the architecture accommodate? | Quoting the empty-slot potential as if it were installed |
| Installed capacity | How much module capacity is present now? | Treating the sum as N+1 protected capacity |
| Protected capacity | How much load can be served after the stated reserve? | Applying N+1 without identifying module size and quantity |
| Runtime | How long can the battery support a defined load? | Assuming more module capacity creates more battery energy |
A buyer should be able to reconstruct the arithmetic from the quote. “60 kW modular UPS” is incomplete if the unit ships with two 20 kW modules. It might mean a 60 kW frame with 40 kW installed. If N+1 is required, only 20 kW may be available to the protected load until a third module is added.
Current XPC architectures illustrate the boundary
The XPC M90S-6S is a single-phase modular online UPS offered in a 6 kW to 24 kW architecture. It grows in 6 kW power-module increments and supports N+1 configurations [1]. The XPC M90U is a different architecture: 208/120 V three-phase, three power-module slots, and 15 kW or 20 kW module options. XPC publishes 15 kW to 60 kW configurations for the family [2].
Those facts must not be blended. A 6 kW module belongs to the M90S-6S architecture. The M90U uses 15 kW or 20 kW modules. Voltage, phase and module design establish the product boundary before any arithmetic begins.
Figure 2. Module arithmetic is product-specific. Graphic by GDF Technologies from current XPC product information [1][2].
Calculating N+1 protected capacity
For equal power modules, a simple module-level N+1 configuration reserves one module. With three 15 kW M90U modules, 45 kW is installed and 30 kW remains available after reserving one 15 kW module. With three 20 kW modules, 60 kW is installed and 40 kW remains after reserving one 20 kW module.
Figure 3. Published module sizes allow transparent arithmetic. The figure does not replace model-specific design review [2].
| M90U example | Installed modules | Installed capacity | N+1 protected capacity | Unused slot |
|---|---|---|---|---|
| 15 kW modules | 3 | 45 kW | 30 kW | 0 |
| 20 kW modules | 3 | 60 kW | 40 kW | 0 |
| 15 kW modules | 2 | 30 kW | 15 kW | 1 |
| 20 kW modules | 2 | 40 kW | 20 kW | 1 |
The table is module arithmetic, not a complete system rating. Overload behaviour, bypass limits, battery discharge capability, ambient conditions and manufacturer configuration rules still apply. Redundancy also needs an operating definition. N+1 at the module level does not by itself provide a redundant input source, output path, bypass path, battery system or distribution system.
Runtime remains a separate calculation
Power modules convert energy. Batteries store it. Adding a module may increase the UPS power ceiling without increasing runtime. It can even reduce runtime if the added module enables more load while the battery configuration remains unchanged.
Runtime qualification should state:
- protected load in kW and kVA;
- required duration and operating sequence;
- DC configuration and approved battery option;
- battery age, temperature and design margin;
- manufacturer calculation basis and endpoint;
- recharge expectation after the discharge;
- whether one battery path must be unavailable without losing the objective.
The M90U product information describes internal VRLA battery-module options and configurable charge current. Those features create choices, not a universal runtime. A site-specific calculation is still required [2][3].
Selection sequence
Voltage and phase come first. The protected load then defines the required power duty. The resilience objective determines how much installed capacity must be held in reserve. Battery design follows the runtime duty. Site integration confirms that the selected arrangement can be installed, bypassed, cooled, protected and serviced.
Figure 4. The product follows the operating requirement. Graphic by GDF Technologies [1][2][3].
| Decision | Minimum input | Output to record |
|---|---|---|
| Electrical architecture | Input and output voltage, phase, grounding and source | Eligible product family |
| Module quantity | Qualified load, overload and growth | Installed kW and number of modules |
| Redundancy | Failure tolerance and maintenance objective | Protected kW and reserve module count |
| Battery | Runtime load and duration | Battery configuration and calculation reference |
| Bypass | Operating and maintenance states | Bypass rating, source and procedure basis |
| Expansion | Forecast load and timing | Reserved slots, electrical allowance and review trigger |
Expansion is not automatic
An empty slot is useful only if the future module remains supported, the electrical infrastructure has capacity, the battery duty remains acceptable and the added load fits the redundancy objective. Firmware, module compatibility, commissioning requirements and supplier availability can also matter. Expansion should be handled as a controlled design change.
Figure 5. Empty slots create an option, not a completed future design. Graphic by GDF Technologies [2][3].
Before ordering an additional module, confirm the exact model and revision, current installed configuration, supported mixing rules, input and output capacity, bypass duty, battery runtime at the new load, cooling, clearances and the required commissioning steps. Update the single-line diagram, asset record and redundancy statement after the change.
Writing a quote that cannot be misread
A clear modular UPS line item should state the frame, module size, module count, installed capacity, protected capacity under the stated redundancy, voltage and phase, battery configuration, runtime basis, bypass scope and expansion assumptions. If any value is provisional, identify the release condition.
GDF Technologies can prepare modular UPS designs and quotes using that structure. See GDF Technologies’ Xtreme Power Conversion services for service context. Final selections require current XPC documentation and site-specific engineering.
References
- Xtreme Power Conversion, M90S-6S product page, accessed July 12, 2026.
- Xtreme Power Conversion, M90U product page, accessed July 12, 2026.
- Xtreme Power Conversion, M90U User Manual, 2025.
- GDF Technologies, GDF Field Case Library: Engineering and Quoting Patterns, anonymized internal record, 2026.
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