How to Select and Size an Online UPS for Unattended Network Closets

Network switches, routers, firewalls, wireless access points, and security cameras usually sit in network closets, branch offices, and edge sites with no IT staff on site. The UPS at these sites has to carry the load through outages and poor power quality without anyone in person to intervene. That depends on choosing the right topology, sizing the UPS in watts against a load that grows with PoE, and setting a runtime target.

This article explains how to choose a UPS topology for an unattended site and how to size it in watts and runtime against the real network load. It then shows how the Eaton Tripp Lite series SmartOnline double-conversion UPS family addresses each of these decisions, from compact tower units to 2U rack/tower models for 120 V and 208/230 V closets.

Why topology matters at an unattended site

A line-interactive UPS passes utility power to the load during normal operation. An autotransformer is usually used to adjust any moderate sags and swells. When the input fails, the inverter takes over after a short transfer time.

An online double-conversion UPS instead converts incoming AC to DC and continuously rebuilds a regulated sine wave from that DC bus, so there is no transfer time when the utility fails. Both topologies typically include surge suppression and line noise filtering, but an online UPS also isolates the load from voltage irregularities and frequency variations.

Figure 1 shows a typical online double-conversion power path. Incoming power passes through an input filter and an AC/DC stage to a DC bus, and a DC/AC inverter feeds the load through an output filter. The battery connects on the DC side through a charger and a DC/DC converter, so an outage changes the energy source without interrupting the inverter output. A separate bypass path carries the load only during an overload or an internal fault.

Figure 1: Block diagram of an online double-conversion UPS, showing the AC/DC stage and DC/AC inverter in the main power path, the battery charger and the DC/DC converter on the DC side, and the bypass path. (Image source: Eaton)

But this continuous conversion comes at a cost. The AC/DC stage and inverter dissipate power all the time, so an online UPS typically runs at lower efficiency than a line-interactive unit carrying the same load. For a small closet with stable utility power, a line-interactive UPS is often a reasonable choice. Online double conversion fits sites with poor or unknown power quality, frequent generator transfers, or no technician available to reset equipment after a power cut.

Sizing the load and setting a runtime target

UPS sizing starts with the equipment that keeps the site connected, including access and aggregation switches, the router and firewall, and any media converters on fiber links. Power over Ethernet (PoE) switches need particular attention because their draw includes the power delivered to access points, cameras, and phones. Sizing based on the actual or planned PoE load helps avoid an undersized UPS.

Many online UPS models are rated at an output power factor of 0.9, so a 1,500 VA unit supports up to 1,350 W. IT power supplies with power factor correction bring VA demand close to watt demand, so the watt rating is usually the limit to check first, although the UPS must still satisfy both its watt and VA ratings.

The runtime target depends on whether the site must ride through brief outages, bridge to a standby generator, or keep the core switch and uplink running longer. Runtime does not scale linearly with load, because a battery delivers less usable capacity at high discharge rates. Reducing the load extends runtime by more than the load ratio alone suggests, and external battery modules extend it further. Consideration should also be given to the aging of VRLA batteries.

How the SmartOnline UPS family addresses each decision

Continuous conditioning with zero transfer time: The SmartOnline UPS system from Eaton Tripp Lite uses online double conversion with a 0 ms transfer time between line and battery modes. Figure 2 contrasts this behavior with a line-interactive UPS, whose output pauses briefly while it transfers to the battery, typically for 2 to 4 ms. If an overload or internal fault occurs, the UPS transfers the load to bypass, which keeps the equipment running on utility power without battery backup.

Figure 2: When input power fails, the output of an online UPS continues without interruption, while a line-interactive UPS output pauses briefly during its transfer to battery. (Image source: Eaton)

Load shedding and extended runtime: Each model has programmable load banks that can be switched off to shed non-critical equipment, while the primary outlets keep the core switch, router, and uplink powered. Up to four matching external battery modules connect through a dedicated connector, and a detection port lets the UPS recognize them.

Eaton’s runtime chart for one of their models shows the result in Table 1. The internal batteries carry the full 1,350 W load for 5.8 minutes, roughly half the load for 16 minutes, and a 200 W load for 64 minutes. Halving the load nearly triples the runtime rather than doubling it. One external module raises full-load runtime to 27 minutes, and four modules extend it to 100 minutes.

Table 1: Estimated runtime of the SU1500RTXLCD2U in minutes versus connected load in watts, with internal batteries and with one to four matching external battery modules. (Image source: Eaton)

Automatic recovery and remote access: The UPS restarts automatically when utility power returns, even after a complete battery discharge. The optional WEBCARDLXE network management card, shown in Figure 3, adds SNMP, web, and SSH monitoring and automated alerts. It can also power-cycle an unresponsive switch or router via the programmable load banks, either on command or automatically when the card detects a loss of connectivity.

Figure 3: The WEBCARDLXE network management card, which installs in the accessory slot of compatible SmartOnline UPS models. (Image source: Eaton)

Matching a SmartOnline model to the site

The four models below span the range from a small site with no rack to a 208 V server room or industrial cabinet. Each supports up to four external battery modules and includes programmable load banks.

SU1000XLCD, 1,000 VA (900 W): This tower UPS, pictured in Figure 4, sits on a shelf or a floor in a small branch office or retail backroom, where it protects the access switch, router, and firewall of a small network. It draws 9 A at maximum load from a NEMA 5-15P plug and provides six 5-15R outlets, including two switchable load banks of two outlets each. It dissipates 419 BTU/hr (123 W) in online mode at full load and weighs 32.63 lb.

Figure 4: Front and rear views of the SU1000XLCD tower UPS, showing the LCD interface, the optional network management card slot, the communication ports, and six 5-15R outlets. (Image source: Eaton)

SU1500RTXLCD2U, 1,500 VA (1,350 W): This 2U rack/tower model, shown front and rear in Figure 5, mounts in a 19-inch 4-post rack in an intermediate distribution frame (IDF) closet or edge network site to support access switches, router, and uplink equipment up to its 1350 W rating. It draws 12.5 A at maximum load from a NEMA 5-15P plug and provides eight 5-15R outlets, four of them in two programmable load banks. It dissipates 512 BTU/hr (150 W) in online mode at full load, and weighs 42.55 lb.

Figure 5: Front and rear views of the SU1500RTXLCD2U 2U rack/tower UPS, showing the LCD interface, the optional network management card slot, the communication ports, and eight 5-15R outlets. (Image source: Eaton)

SU2200RTXLCD2U, 2,000 VA (1,800 W): This model, seen in Figure 6, serves denser closets where high-wattage PoE switches feed access points, cameras, and phones, but the site has only 120 V service. It draws 16 A at maximum load from a NEMA 5-20P plug, which requires a 20 A circuit, and provides six 5-20R outlets and one L5-20R outlet. It dissipates 578 BTU/hr (169 W) in online mode at full load and weighs 61.51 lb in a 2U rack/tower chassis.

Figure 6: Front and rear views of the SU2200RTXLCD2U, with its NEMA 5-20P input cord, six 5-20R outlets, and one L5-20R outlet. (Image source: Eaton)

SUINT2200LCD2U, 2200 VA (2000 W): The 200 V to 240 V model in Figure 7 fits server rooms, industrial control cabinets, and other sites with 208 V or 230 V service. Its 296 J surge suppression rating is higher than the 190 J of the 120 V models featured here. It draws 10.57 A at 208 V through a C20 inlet with a C19 to L6-20P cord and provides eight C13 and two C19 outlets in primary and switchable groups. It dissipates 513 BTU/hr (150 W) in online mode at full load, and weighs 59.75 lb.

Figure 7: Front and rear views of the SUINT2200LCD2U, with its C20 power inlet and eight C13 and two C19 outlets in two groups. (Image source: Eaton)

All four models accept the optional WEBCARDLXE network management card in their accessory slot. The SU1500RTXLCDN and SU2200RTXLCDN rack/tower models ship with the WEBCARDLXE preinstalled. Apart from the card, they match the SU1500RTXLCD2U and SU2200RTXLCD2U in ratings, input plugs, and outlets.

Conclusion

A UPS for an unattended network closet must keep the site connected without requiring anyone to physically reset the equipment. That calls for a topology matched to the power quality at the site, a rating checked in watts against the real load, including PoE, and a runtime target with margin for battery aging.

The Eaton Tripp Lite series SmartOnline family meets these requirements with online double conversion and 0 ms transfer, programmable load banks, expandable runtime, and remote management through an optional network card. The four models featured here run from a 1,000 VA tower for small sites to a 2,200 VA rack/tower model for 200 V to 240 V service, and the family extends below and above that range. So, a single product family can cover the closets across a distributed network.