UPS Buying Guide: How to Choose a UPS for Your Server Rack

Posted by Robert Faulkner on September 15, 2022

Availability is the name of the game when it comes to IT systems. While it’s impossible to predict, much less prevent, every unplanned outage, organizations can take steps to reduce the risk. That’s why uninterruptible power supplies (UPS) are essential IT infrastructure components across businesses of all sizes.

A UPS traditionally provides two things: 

  1. Battery backup power if the primary power source is unavailable.
  2. Power conditioning to protect critical IT equipment from power surges, sags, and other miscellaneous fluctuations. 

They have built-in mechanisms for detecting power outages and voltage drops and rapidly switching to battery backup. Beyond that basic definition, UPSs vary widely in terms of:

  • Footprint / space requirements
  • Battery runtime
  • Lifespan of the battery
  • Battery recharge time and cycle life
  • Power, cooling, and other operating costs
  • Energy usage
  • Operational overhead

The power load the UPS is supporting has a lot of influence on the differences between units. A single rack-mounted UPS could keep equipment running for an hour or more for the lowest-density racks. However, in the data center context, the power load is so high that operators can’t rely on UPSs alone. They typically need their UPS to run just long enough for generator power to come online. At a minimum, the UPS needs to provide enough power to ensure the safe shutdown of systems. Although, most data centers today can’t afford a millisecond of downtime

The capital and operating costs of uninterruptible power supplies often represent significant line items in the IT budget. This buying guide offers criteria for evaluating UPSs to help you get the most value for your investment.

What Size UPS Do I Need?

The size and weight of a UPS is primarily dependent on power requirements. Data center UPSs often look like standard 42U racks because of the enormous loads they’re expected to support, while traditional network closets and server rooms typically only need one or two 2U rack-mounted UPS. The type of battery also influences the UPS’ size and weight; lead-acid batteries are heavier than lithium (lithium batteries vs. lead-acid batteries). The factors below directly influence UPS size and weight.

  • Capacity. To determine the capacity, calculate the maximum watt and volt-ampere (VA) ratings of all the equipment the UPS is to support. The UPS should have watt and VA ratings higher than the total load. The output watt capacity should be 20 percent to 25 percent more than the total power drawn by the equipment.
  • Form Factor. Most UPSs have one of two form factors — tower (freestanding) or rack mount. Tower UPSs stand on the floor or a shelf while rack mount units mount inside the rack along with the IT equipment. Rack mount units typically occupy 2U to 3U of vertical space. Tower units can be as small as a space heater or as large as a full-size rack.
  • Weight. UPSs can be heavy, and lead-acid UPSs are much heavier than lithium-ion UPSs (lithium UPS vs. lead-acid UPS). If they are rack-mounted, it’s important to ensure that the rack has the load capacity to support the weight and mount them in the lower part of the rack. Manufacturers typically provide documentation on the UPS's recommended position.

Do I Need a Rack Mount UPS?

Rack mount UPSs are designed to fit within the standard 19-inch racks commonly used in data centers, server rooms, network closets, and edge environments. They come in a range of sizes up to 10 kVA+. Rack mount units simplify maintenance and management, helping to maximize uptime and business continuity. Furthermore, rack mount units eliminate the single point of failure associated with one larger UPS. After all, redundancy is the magic word for network design. In the era of 24/7 connectivity, organizations can’t spare a second of downtime. 

Factors to Consider When Choosing a UPS

Purchase price is just one factor in the total cost of ownership (TCO) of a UPS. Organizations should consider these eight factors before making a purchasing decision.

Battery Topology

There are three common UPS topologies: offline (standby), line-interactive, and online (double-conversion). The most basic, cheapest option is offline (standby), and the most advanced, premium option is online (double-conversion). In a separate piece (linked above), we dive more into the differences between online, line-interactive, and offline UPSs, but the main difference concerns power conditioning. Online UPSs provide the maximum level of protection against power sags, spikes, and under/over voltage.

Battery Life

While the lifespan of a UPS is about ten years, some batteries must be replaced after just three to five years. The cost of additional batteries and the labor involved in replacing them adds to the TCO calculation. There may also be battery disposal costs.

Recharge Time and Cycle Life

Traditional valve-regulated lead-acid (VRLA) batteries require up to 24 hours to fully recharge, while lithium-ion batteries recharge in just two hours. VRLA batteries typically provide 200 discharge cycles, meaning you can deplete and recharge the battery 200 times before it fails. Lithium-ion batteries offer 500 to 7,000 discharge cycles depending upon the chemistry and have a slow rate of discharge when not in use. 

Operating Temperature

One of the biggest factors affecting battery life is operating temperature. The recommended operating temperature for VRLA batteries is 68 degrees to 77 degrees Fahrenheit, while lithium-ion UPSs can operate at up to 140 degrees Fahrenheit without affecting the battery's lifespan. The need to cool a UPS adds to data center operational costs and thus the TCO of the UPS. Some battery compositions, such as nickel-cadmium, are designed to operate effectively in high-heat environments.

Energy Usage

UPS batteries are “trickle charged” — enough electricity is supplied to offset the battery’s self-discharge rate and keep the battery fully charged. Some batteries lose more energy during trickle charging than others and thus consume more electricity during “float” or standby mode. “Load shedding” allows users to power down noncritical receptacles, leaving the receptacles with critical equipment operational.

Operational Overhead

An uninterruptible power supply with a built-in battery management system (BMS) provides status and fault monitoring, cell balancing, power optimization, and more, reducing the labor required for maintenance. It also helps reduce the risk of system downtime due to battery failure. 

Remote Monitoring and Management

Best-in-class UPSs provide network connectivity for remote monitoring and management. This simplifies maintenance, thereby reducing TCO.

Sine Wave

The sine wave refers to the waveform of the battery output. Some UPSs produce a pure sine wave, while others produce a simulated or modified sine wave. Because a pure sine wave is smoother and cleaner, it is preferred for critical and highly sensitive equipment. A modified sine wave provides rougher, less stable output but is adequate for PCs, A/V components, and other less-critical equipment. Online UPSs always produce a pure sine wave, but line-interactive and offline UPSs sometimes produce a modified sine wave.

Auto Restart / Manual Restart

When a UPS runs out of power, a technician typically has to go to the rack and manually reboot the UPS once primary power is restored. A UPS with an auto-restart feature does this automatically, reducing labor costs.

Best Rack Mount UPSs from Enconnex

Enconnex offers a complete line of rack mount UPSs, including line-interactive and online (double-conversion) topologies with lead-acid and lithium-ion (LiFePO4) batteries. Power capacities range from 800 VA to 10 kVA to support a wide range of rack densities. All models provide pure sine wave output for maximum power reliability. Our UPSs offer industry-leading features, including smart displays for precise monitoring, hot-swappable battery packs, auto-restart, and more. Customers can bundle Enconnex UPSs with our line of EdgeRack micro data center cabinets for a complete, prefabricated solution. Contact one of our specialists for help choosing the right UPSs for your application.

Posted by Robert Faulkner on September 15, 2022

Robert Faulkner is the Vice President of Engineering and Operations at Enconnex. He comes from a strong background in product management with over 20 years in the IT industry. He currently holds an MSME and CDCD certification. He earned his MS degree in Mechanical Engineering at University of Nevada, Reno.

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