Power over Ethernet (PoE) technology has revolutionized network infrastructure by allowing both data and power to be transmitted over the same Ethernet cable. PoE switches are commonly used in network wiring closets to power devices such as IP phones, wireless access points, and security cameras. However, ensuring reliable power supply for these PoE switches is critical. In this article, we’ll explore the challenges associated with sizing an Uninterruptible Power Supply (UPS) for a network closet with PoE switches and provide best practices for proper sizing

1. Understanding the Power Load

Before sizing a UPS for a network closet with PoE switches, it’s crucial to understand the power load. PoE switches not only handle data traffic but also provide power to connected devices. The power requirements of these devices vary based on their type (e.g., IP phones, cameras, access points). Therefore, the first challenge is accurately assessing the total power consumption.

  • Best PracticeDocument the power requirements of each PoE device in the network closet. Consult the manufacturer’s specifications or use a power meter to measure actual power draw.

2. Calculating the Total Load

Once you have the power requirements for all connected devices, calculate the total load. Consider both active and standby power consumption. Additionally, account for any future expansion (adding more PoE devices).

  • Best PracticeSum up the power requirements of all devices to determine the total load. Add a safety margin (usually 10-20%) to accommodate unexpected spikes or future additions.

3. Runtime Requirements

The UPS should provide sufficient runtime during power outages. The runtime depends on the PoE load and the UPS capacity. Consider how long you need the PoE switches to run during an outage (e.g., until the generator kicks in or until manual intervention).

  • Best PracticeCalculate the required runtime based on your organization’s policies and operational needs. A longer runtime may require a larger UPS.

4. Selecting the Right UPS Capacity

Choosing the correct UPS capacity is critical. Undersized UPS units won’t provide adequate backup time, while oversized units can be costly and inefficient.

  • Best Practice:
    • Match the UPS capacity to the total load calculated earlier.
    • Consider the efficiency curve: UPS systems are most efficient at around 50-75% load. Oversizing can lead to lower efficiency.
    • Look for a UPS with a power factor correction (PFC) feature to handle reactive power from PoE devices.

5. Battery Technology and Maintenance

The type of battery affects UPS performance. Common options include valve-regulated lead-acid (VRLA) and lithium-ion batteries. VRLA batteries are cost-effective but have shorter lifespans. Lithium-ion batteries last longer but are pricier.

  • Best Practice:
    • Choose the right battery type based on your budget and maintenance capabilities.
    • Regularly test and maintain batteries to ensure optimal performance.

6. Physical Space Constraints

Network closets are often tight on space. Ensure the selected UPS fits within the available area. Consider rack-mounted or wall-mounted options.

  • Best PracticeMeasure the available space and choose a UPS that fits without compromising ventilation or accessibility.

7. Redundancy and Scalability

Consider redundancy for critical PoE switches. Implementing dual UPS systems (N+1 redundancy) ensures continuous operation even if one UPS fails. Also, plan for scalability as your network grows.

  • Best Practice:
    • Evaluate redundancy needs based on criticality.
    • Choose a modular UPS system that allows easy expansion.


Sizing a UPS for a network closet with PoE switches requires careful consideration of power load, runtime, capacity, and physical constraints. By following best practices, you can ensure reliable power backup and minimize downtime in critical network environments.

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