UPS Selection Guide

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  1. UPS Selection Guide: Ensuring Data Center Resilience and Uptime

This document serves as a comprehensive technical guide for selecting and integrating Uninterruptible Power Supply (UPS) systems tailored to modern server hardware configurations. Proper UPS sizing and configuration are critical for mitigating power disturbances, ensuring data integrity, and maintaining business continuity in mission-critical environments.

This guide focuses on a high-density, enterprise-grade server platform designed for virtualization and intensive database workloads, providing the necessary context for accurate power protection sizing.

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    1. 1. Hardware Specifications

The foundational element for accurate UPS selection is a precise understanding of the server hardware's power draw, both at idle and under peak load. This section details the specifications of the reference server platform, the **"Atlas-9000 Enterprise Compute Node"**, which necessitates robust power conditioning and backup.

      1. 1.1. Atlas-9000 Server Platform Overview

The Atlas-9000 is a 2U rackmount server engineered for maximum compute density and I/O throughput. Its power requirements are substantial due to the integration of high-TDP CPUs and numerous high-speed peripheral components.

Atlas-9000 Core Component Specifications
Component Specification Detail Power Draw Estimation (TDP/Typical)
Chassis 2U Rackmount, Redundant PSU Bays (2x) N/A
Processors (CPUs) 2x Intel Xeon Scalable (Sapphire Rapids), 56 Cores each (Total 112 Cores) 2x 350W TDP (Peak: 750W combined)
System Memory (RAM) 2TB DDR5 ECC Registered (48x 64GB DIMMs @ 4800MT/s) ~250W (Peak Load)
Primary Storage Array 8x 3.84TB NVMe U.2 SSDs (PCIe Gen 5) ~60W
Network Interface Cards (NICs) 2x 100GbE ConnectX-7, 1x Dedicated IPMI/Management ~30W
Expansion Cards (Accelerators) 2x NVIDIA H100 SXM5 (Passive Cooling) 2x 700W TDP (Peak: 1400W combined)
Total Peak Theoretical Draw (Excluding PSU Overhead) --- Approximately 2490 Watts
      1. 1.2. Power Supply Unit (PSU) Configuration

The efficiency and redundancy of the PSUs significantly impact the required VA rating of the connected UPS. The Atlas-9000 utilizes Platinum-rated, hot-swappable, redundant power supplies.

Atlas-9000 PSU Configuration
Parameter Value Notes
PSU Rating (Per Unit) 2200W AC Input, 2000W DC Output 80 PLUS Platinum Certified
PSU Efficiency (at 50% Load) 94% Critical for calculating true input power requirements
Redundancy Mode 1+1 (N+1) Only one PSU needs to be active to run the server, but two are required for full redundancy.
Input Power Factor (PFC) >0.98 (Active PFC) Reduces reactive power demand on the UPS.
      1. 1.3. Calculating Nominal Power Consumption

For accurate UPS sizing, we must calculate the *maximum sustained input power* required by the system, accounting for PSU efficiency and redundancy.

    • Scenario:** Server operating at 90% component utilization (high load, but not stress testing) with both PSUs active (N+1 redundancy).

1. **Component Power Draw (Estimated Peak):** 2490 W (from 1.1) 2. **Required DC Output:** 2490 W 3. **Required AC Input (Accounting for 94% Efficiency):**

   $$ \text{AC Input} = \frac{\text{DC Output}}{\text{Efficiency}} = \frac{2490 \text{ W}}{0.94} \approx 2649 \text{ Watts} $$

4. **Total AC Input (Accounting for Redundancy):** Since both PSUs are drawing power in N+1 mode, the total load drawn from the UPS is the sum of the two PSU inputs, which, in a balanced scenario, will be close to the calculated requirement: **~2650 Watts.**

  • Note: When sizing the UPS, the actual power draw must be multiplied by a safety margin (typically 1.25) and converted to VA, considering the system's Power Factor (PF).*

$$ \text{Apparent Power (VA)} = \frac{\text{Real Power (W)}}{\text{Power Factor (PF)}} $$ $$ \text{VA} = \frac{2650 \text{ W}}{0.98} \approx 2704 \text{ VA} $$

For operational headroom and to support future component upgrades (e.g., liquid cooling integration), a minimum UPS capacity of **3500 VA** is recommended for this single node. For a rack containing four such nodes, a **15 kVA** or larger three-phase UPS system would be necessary.

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    1. 2. Performance Characteristics

The primary role of the UPS is not just to provide runtime, but to deliver **clean, conditioned power** that prevents transient events from affecting sensitive server components. This section details the UPS performance metrics crucial for high-reliability computing.

      1. 2.1. Topology Selection and Output Quality

The choice of UPS topology directly dictates the power quality delivered to the Atlas-9000.

  • **Standby/Line-Interactive:** Unsuitable. Insufficient switching time and poor regulation for high-density GPU/CPU systems.
  • **Double Conversion Online (VFI):** Mandatory for this density. The constant regeneration of AC power isolates the load from all input anomalies.
Online UPS Performance Metrics
Parameter Specification Requirement Impact on Server Hardware
Transfer Time (Utility to Battery) 0 ms (Zero Transfer Time) Essential for preventing server reboots during minor brownouts or frequency shifts.
Output Total Harmonic Distortion (THD) < 3% (Linear Load) Minimizes heating and stress on PSU input capacitors and PFC circuits.
Output Frequency Stability $\pm 0.1 \text{ Hz}$ Critical for maintaining the stability of server clock sources and high-speed bus timings.
Output Voltage Regulation $\pm 1\%$ (Online Mode) Ensures stable voltage delivery under rapid load fluctuations (e.g., accelerator burst workloads).
      1. 2.2. Runtime Benchmarking (3500 VA UPS Example)

Runtime is dictated by the battery chemistry (typically Valve Regulated Lead Acid - VRLA, or Lithium-Ion - Li-Ion) and the load percentage. For the Atlas-9000 drawing 2704W (approx. 77% load on a 3500 VA unit), performance varies significantly based on battery type.

| Load Percentage | Real Power Draw (W) | VRLA Runtime (minutes) | Li-Ion Runtime (minutes) | | :---: | :---: | :---: | :---: | | 100% | 3500 VA (approx. 3300W) | 3.5 | 5.0 | | 77% (Atlas-9000 Peak) | 2704W | 5.5 | 8.5 | | 50% (Idle/Light Load) | 1750W | 10.0 | 18.0 |

  • Note: These figures assume a standard configuration of internal battery modules. Extended runtime modules (ERMs) will scale these figures linearly.*
      1. 2.3. Overload Capacity and Short-Circuit Handling

Server PSUs, especially those with Active PFC, can present high inrush currents upon startup or during severe load step-changes. The UPS must handle these transients without tripping its own protection circuits.

  • **Sustained Overload:** The UPS must support **125% load for 10 minutes**. This allows for controlled shutdown sequences even if one server component faults and draws excessive power momentarily.
  • **Short-Circuit Handling:** The UPS must be capable of clearing a short circuit on the output line within **10 milliseconds** to protect the server's internal power distribution board (PDB) before the server's own internal fuses activate, potentially causing cascading failure. PDUs must also be coordinated with the UPS trip curve.

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    1. 3. Recommended Use Cases

The Atlas-9000, combined with a high-quality Online UPS, is optimized for environments where data integrity and continuous operation justify the higher capital expenditure (CAPEX) associated with enterprise-grade power protection.

      1. 3.1. High-Performance Computing (HPC) and AI Training Clusters

The inclusion of multiple high-TDP accelerators (like the H100) means that power fluctuations can cause immediate throttling or complete cluster failure.

  • **Requirement:** Instantaneous power delivery during node synchronization or burst compute cycles.
  • **UPS Role:** The Online topology ensures that the accelerators receive perfectly stable power, maximizing their utilization rates ($>95\%$). The defined runtime (5-8 minutes) is usually sufficient for the Cluster Scheduler to gracefully quiesce active jobs and store intermediate states to persistent storage area networks (SAN) before a complete shutdown.
      1. 3.2. Mission-Critical Database Servers (OLTP/OLAP)

Database systems are exceptionally sensitive to I/O corruption caused by sudden power loss. Even a fraction of a second of lost power can cause transaction log inconsistencies requiring lengthy recovery procedures (e.g., Oracle RMAN recovery).

  • **Requirement:** Zero data loss and minimal recovery time objective (RTO).
  • **UPS Role:** The UPS maintains power long enough for the database engine to flush all pending memory writes (dirty buffers) to the non-volatile storage layer. For extremely low RTO environments, dedicated energy storage for the storage array is often layered on top of the main server UPS.
      1. 3.3. Virtualization Hypervisors (Primary Management Nodes)

In large virtualization farms, the hypervisor hosts (e.g., VMware ESXi, KVM) are the control plane for hundreds of virtual machines (VMs). A failure here cripples the entire infrastructure.

  • **Requirement:** Sustained uptime for orderly VM migration (vMotion/Live Migration) or graceful shutdown orchestration.
  • **UPS Role:** Provides enough time (the 5-8 minute window) for the management suite to execute shutdown scripts, preventing the "hard crash" of guest operating systems.

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    1. 4. Comparison with Similar Configurations

Selecting the correct UPS capacity requires comparing the target load against available commercial off-the-shelf (COTS) solutions and assessing the trade-offs between runtime and physical footprint.

      1. 4.1. UPS Sizing Comparison for a Single Atlas-9000 Node (2704W Load)

| UPS Capacity (kVA) | Topology | Estimated Cost Index ($) | Runtime at 2.7kW Load (VRLA) | Footprint (U Height) | Suitability for Atlas-9000 | | :---: | :---: | :---: | :---: | :---: | :---: | | 3 kVA | Online (VFI) | 2.5 | 6 minutes | 2U | Minimum Viable (No Headroom) | | **5 kVA** | **Online (VFI)** | **3.5** | **11 minutes** | **3U** | **Recommended Standard** | | 10 kVA | Online (VFI) | 5.0 | 22 minutes | 6U | Over-provisioned for single node; good for 2 nodes | | 3 kVA | Line-Interactive (VI) | 1.5 | 8 minutes | 2U | Unacceptable Power Quality |

      1. 4.2. Runtime Strategy Comparison

The decision between purchasing a larger UPS upfront (oversizing) versus adding External Battery Modules (EBMs) later involves balancing initial cost against long-term flexibility and physical space management.

Runtime Strategy Trade-offs
Strategy Pros Cons Best Suited For
**Strategy A: Oversizing (e.g., 10kVA for a 3kVA load)** Excellent long runtime; lower component stress (runs at lower load %); simpler management. Higher initial CAPEX; larger physical footprint (more U space); wasted capacity if load does not grow. Environments requiring guaranteed >20 minute runtime without EBMs. High power density racks.
**Strategy B: Modular Expansion (e.g., 5kVA base + EBMs)** Lower initial cost; scalable capacity on demand; smaller initial footprint. EBMs add complexity and require additional rack space later; runtime calculation needs careful tracking of added modules. Environments with uncertain short-term load growth but predictable long-term expansion.
**Strategy C: Lithium-Ion Batteries** Significantly smaller physical size for equivalent runtime; longer cycle life; lower maintenance costs. Higher initial cost per kWh; thermal management is more critical; regulatory concerns in some regions. Space-constrained edge deployments or high-density core data centers. See Li-Ion technical deep dive.
      1. 4.3. Comparison with Tiered Protection Schemes

For extremely large deployments, protecting every watt of power with a high-capacity UPS may be cost-prohibitive. A tiered approach is often used:

1. **Tier 1 (Critical):** Management servers, storage controllers, core network switches. Protected by **Online UPS** with 15-30 minute runtime. 2. **Tier 2 (Essential Compute):** Atlas-9000 nodes. Protected by **Online UPS** with 5-10 minute runtime (enough for graceful shutdown). 3. **Tier 3 (Non-Essential/Batch):** Test/Dev servers, monitoring infrastructure. Protected by **Line-Interactive UPS** or direct generator feed.

The Atlas-9000 configuration discussed here falls squarely into Tier 1/Tier 2 requirements, mandating the Double Conversion Online topology. Generator synchronization must be tested rigorously when using Tier 2 systems, ensuring the UPS can seamlessly transfer from battery to generator power.

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    1. 5. Maintenance Considerations

The reliability of the UPS system is directly proportional to the rigor of its maintenance schedule. Neglecting maintenance leads to battery failure, which is the single leading cause of UPS system failure during an actual outage.

      1. 5.1. Battery Management and Replacement Cycles

The lifespan of VRLA batteries is highly dependent on ambient temperature.

  • **Temperature Derating:** For every $8^{\circ}\text{C}$ increase above the baseline $25^{\circ}\text{C}$, the expected battery life is halved. A UPS operating in a $35^{\circ}\text{C}$ environment will see its 5-year battery life reduced to approximately 2.5 years. Adherence to ASHRAE thermal standards is paramount.
  • **Testing:** Automated self-tests are standard, but **monthly manual load discharge tests** (simulating a utility failure for 5 minutes) are necessary to verify battery capacity under load conditions specific to the Atlas-9000's high-current draw.
  • **Replacement:** VRLA batteries should be proactively replaced every 3 to 5 years, regardless of testing results, as capacity degradation is non-linear towards the end of life.
      1. 5.2. Power Quality Monitoring and Logging

Modern UPS systems must integrate deeply with the server management infrastructure via SNMP or Modbus TCP/IP.

  • **Event Logging:** The UPS management card must log all power events (sags, swells, outages) with microsecond precision. This data must be correlated with server performance logs (e.g., IPMI/Redfish logs) to diagnose intermittent brownout-related instability that might falsely appear as a CPU or RAM error.
  • **Predictive Failure Analysis:** Monitoring battery impedance trends allows for the prediction of battery end-of-life before a catastrophic failure during an outage. A sudden drop in impedance across multiple strings is a strong indicator of imminent failure. Best practices for SNMP polling must be established.
      1. 5.3. Thermal Management and Airflow

The UPS itself generates significant heat, which must be accounted for in the overall data center cooling budget.

  • **Heat Rejection:** An Online UPS running at 77% load (2.7kW) will reject approximately 200W to 300W of heat into the room, depending on efficiency. This heat load must be included in the Computer Room Air Handler (CRAH) capacity planning.
  • **UPS Placement:** UPS units should be placed in dedicated, temperature-controlled zones, ideally immediately adjacent to the IT racks they serve, to minimize cable runs and voltage drop. They should never be placed directly under hot exhaust aisles without adequate baffling, as this can lead to thermal runaway in the batteries.
      1. 5.4. Maintenance Contracts and Service Level Agreements (SLAs)

Given the mission-critical nature of the Atlas-9000, the UPS should be covered by a comprehensive, 24/7/365 maintenance contract.

  • **Response Time:** The SLA for on-site service for critical failures (e.g., inverter failure, major battery fault) must be **less than 4 hours**, matching the typical response time for Tier 1 network hardware.
  • **Spares Management:** The service provider must guarantee local stocking of common replacement parts, specifically the battery strings for the selected 5 kVA model, to ensure rapid swap-out during scheduled preventative maintenance.

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    1. Summary and Conclusion

Selecting the appropriate UPS for a high-density compute node like the Atlas-9000 is a complex engineering task that moves beyond simple VA matching. It requires a deep understanding of the server's power factor, PSU efficiency curves, and the required output waveform quality.

The mandatory configuration for this platform is a **Double Conversion Online (VFI) UPS**, rated for a minimum of **5 kVA**, utilizing high-quality VRLA or Li-Ion batteries to ensure **zero transfer time** and **low output THD** ($\le 3\%$). Adherence to strict maintenance protocols, particularly regarding battery replacement schedules dictated by ambient temperature, is the final guarantor of uptime.

Further reading on redundancy models is recommended before finalizing the power architecture.

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