What Size UPS Do I Need for a Server Rack? VA, Watts and Runtime

The UPS on a server rack has one job: keep the equipment running through a power event long enough for something better to happen, whether that is utility power returning, a generator taking the load, or an orderly shutdown. Sizing it is a short calculation, but most mistakes come from reading the wrong number on the box. The UPS also has to suit the circuit it plugs into and the outlets your equipment uses, which is where the second round of mistakes happens. Here is the method we use, with a worked example.

What size UPS do I need for my server rack?

Work in four steps. First, list every device the UPS will feed and find its real power draw in watts. Second, add them up. Third, add headroom so the UPS is not running at its limit. Fourth, choose battery capacity for the runtime you need, then check the VA rating, input plug and outlets.

• Use measured draw, not power supply nameplates. A server with two 800W supplies does not draw 1,600W. Read actual power from the server's management controller (iDRAC, iLO or IPMI), a metered PDU, or the vendor's power calculator for the exact configuration.
• Measure at realistic peak, not idle. Servers under full CPU or GPU load can draw far more than at rest, and the UPS has to cover the peak.
• Include everything on the circuit: switches, firewalls, storage, KVM and out-of-band management gear. Leave out anything that does not need protection.
• Plan for growth. If the rack will gain servers within the UPS's service life, size for that now or plan a second unit.

How do I calculate it? A worked example

The numbers below are an illustration only, not the draw of any particular product. Suppose a small rack holds two 1U application servers measured at 350W each under load, one storage server at 450W, a pair of switches at 150W combined and a firewall at 50W. The total is 1,350W.

• Measured load (illustration): 350W + 350W + 450W + 150W + 50W = 1,350W.
• Apply headroom: keeping the load at or below about 80% of the UPS watt rating means 1,350W / 0.8 = about 1,690W of UPS capacity.
• Check VA: if the candidate UPS has a power factor of 0.9, 1,690W / 0.9 = about 1,880VA. The UPS must meet both the watt and the VA figure.
• Result: a 1500VA unit rated 1350W would run at 100% and is too small. A 2700W / 3000VA unit such as the APC SRTL3000RM4UXLI, or a 3000W / 3000VA unit such as the APC SRTL3KRM1UIC, carries it with room to grow.
• Growth check (illustration): add another 350W server later and the load becomes 1,700W, about 63% of a 2700W unit and still inside the 80% guideline.

What is the difference between VA and watts on a UPS?

Watts (W) are real power, the energy your equipment actually consumes. Volt-amperes (VA) are apparent power, voltage multiplied by current. The ratio of watts to VA is the power factor. Every UPS carries both ratings, and your load has to fit within both.

Modern server power supplies use active power factor correction, so their power factor is close to 1 and their watts and VA are nearly the same. That makes the UPS watt rating the one that usually runs out first. It is also why the VA number on the front of the box is a poor way to compare units. The APC SMT1500RMI2U is 1500VA and 1000W. The Eaton 5PX 1500VA and Vertiv Liebert GXT5 1500VA are both 1500VA and 1350W. At the top of the range, the APC SRTL3KRM1UIC is rated 3000VA and 3000W. Same badge, different capacity: compare watts. If you are protecting older equipment or devices without power factor correction, the VA figure can become the limit instead, so always check both.

How much runtime does a server UPS need?

Decide what the runtime is for, then size to that:

• Bridge to a generator: runtime has to cover generator start and transfer, plus margin for a failed first start. This is usually minutes, not tens of minutes.
• Graceful shutdown: runtime has to cover the time to detect the outage, wait a set delay in case power returns, then shut down virtual machines, flush storage and power off servers in order. Time an actual test shutdown and add margin. Vendor shutdown software, connected through a network management card or USB, triggers the sequence automatically.
• Ride-through only: for sites that simply need to survive brief dips and interruptions, a few minutes at full load is often enough.

Runtime does not scale in a straight line with load. A UPS at half load typically runs for more than twice as long as at full load, so a larger UPS is sometimes the cheaper way to buy runtime. Use the manufacturer's runtime chart at your actual load in watts. If the internal battery is not enough, many rack models accept external battery packs, listed on our runtime extension page. Battery chemistry matters too: lithium-ion units such as the SRTL3000RM4UXLI are chosen for longer battery service life and lower weight, while traditional sealed lead-acid units remain common. Either way, batteries wear out: plan replacements over the life of the UPS and run periodic self-tests so a weak battery is found before an outage, not during one.

What input voltage and outlets do I need?

In the US, a 120V 15A circuit supports about 1,440W of continuous load (80% of 1,800W) and a 20A circuit about 1,920W. Above that, you need a 30A circuit with a locking receptacle, or a 208V circuit, and your electrician needs to know before the UPS arrives. Many rack UPS models, including the SMT1500RMI2U and SRTL3KRM1UIC, are 230V units intended for 208V or 230V circuits. For a 120V site, the APC Smart-UPS SMT2200RM2UC is a 120V model. Then count outlets by type: the SMT1500RMI2U has four C13 outlets, the Eaton 5PX 1500VA eight C13. Servers with high-wattage supplies may need C19 outlets or a rack PDU fed from the UPS.

When is a rack UPS the wrong answer?

GPU servers change the scale. A single multi-GPU node can draw more than any single-phase rack-mount UPS delivers, and a rack of them moves you into three-phase units such as the APC Easy UPS 3S 20kVA, a 400V double-conversion model, or into facility UPS provided by the data center. Our guides to H100 server power and cooling and AI data center power planning cover rack budgets at that level, and the redundancy and failover guide covers A and B power feeds. Even in a small rack, consider two smaller UPS units on separate circuits feeding the two power supplies of each server: that survives a UPS fault, which one larger UPS cannot.

Size in watts, confirm in VA, buy runtime for a purpose, and never plan to run a UPS at 100%.

Nexus Compute supplies new rack UPS units and battery packs through authorized distribution, with the full manufacturer warranty. Send your equipment list, circuit type and runtime target, and we will recommend a size and quote within 48 business hours.