Which rack equipment needs UPS protection?

Inventory the equipment in the rack and identify which services depend on each item. Include servers, network switches, storage and any other equipment needed to keep essential operations running. Separate these from non-essential loads that don’t need battery backup. This keeps the design focused on what must remain powered during an outage. For each device, record its power supplies and how they’re connected. A server with redundant supplies may have each supply fed from a different power path. That arrangement provides the intended resilience only if the UPS and distribution design preserve the separation. Connecting both supplies to the same protected path can leave a shared point of failure. Map the actual connections rather than relying on equipment labels alone.

Should a rack-mount UPS be sized in watts or volt-amperes?

Check both the watt and VA limits on the UPS, then compare them with the connected equipment’s requirements. The lower applicable limit can constrain the load the unit supports. For example, a rack load may sit below the UPS’s watt rating but exceed its VA rating. In rack mount UPS sizing, don’t treat a VA figure as interchangeable with watts; compare the UPS output specifications with the equipment data.

How much spare capacity should I allow when sizing a rack UPS?

There isn’t one spare-capacity percentage that suits every rack. Base the margin on known equipment additions, realistic demand peaks and uncertainty in the load data. Include a confirmed server addition using its power requirements; a hypothetical future project shouldn’t automatically dictate a much larger UPS. Record what the spare capacity is intended to accommodate, then revisit that assumption if plans change.

Can a rack UPS support equipment with dual power supplies?

Yes, if the UPS arrangement can support the equipment’s load and the power supplies are connected as intended. For example, two supplies may be fed from separate UPS-backed paths so one path can continue supporting the device if the other is lost. If both are connected to the same UPS, that UPS remains a shared point of failure. Map each supply and path before relying on redundancy.

What happens if a rack-mount UPS is undersized?

An undersized UPS may be unable to support the connected load, particularly when demand rises above typical operating levels. Depending on the unit and overload condition, it may raise an alarm or limit its output, transfer to bypass or stop supporting the load. Battery runtime can also fall short of the operational target. Check the UPS’s overload response in its documentation rather than assuming all units behave the same way.

How often should I review my rack UPS sizing?

Review the design whenever a change could affect the load or backup needs, such as adding storage, replacing servers or changing which services must remain available. Include battery condition in routine operational reviews, as battery performance can affect available runtime over time. Keep the current equipment schedule and UPS configuration together so you can assess a proposed change before it becomes part of the live rack.

Does the PDU rating determine the UPS size?

No. A PDU distributes power to connected equipment, while UPS sizing depends on the load and the UPS’s output limits. The PDU’s rating is still an important boundary: the connected load and supply arrangement must remain within its specifications. A high-rated PDU doesn’t mean the UPS can support that full rating. Consider both devices’ limits and the actual equipment load.

Can I increase UPS runtime without changing the rack load?

Possibly. Some UPS configurations support additional battery capacity, depending on the unit and compatible battery options. Confirm expected runtime using the manufacturer’s performance data for the intended UPS, battery configuration and load. Adding battery capacity doesn’t increase the UPS’s output watt or VA limit, so it can extend backup duration only if the existing unit can already support the connected equipment.

Rack-Mount UPS Sizing: Match Capacity to Load and Runtime

UPS Power Systems

Jayden Currie

October 12, 2026

10 minutes

Choosing a rack-mount UPS: matching a RackPower model to your load | RackPower

Choosing a rack-mount UPS: matching a RackPower model to your load

Once you know your rack's measured peak load and how long it needs to stay up, choosing a UPS comes down to four checks: capacity, runtime, power supply, and space in the rack.

The RackPower rack UPS range

Every unit in the range is true online double conversion, rated at power factor 1.0, so the watt rating equals the VA rating.

Model	Rating	Efficiency (online)	Comfortable load (≤ 75%)	Rack height	Input connection	Typical use
USS001BR	1,000 VA / 1,000 W	Up to 96%	750 W	2U	10 A NZ 3-pin plug	Network cabinet, firewall, switches, small NAS
USS002BR	2,000 VA / 2,000 W	Up to 96%	1,500 W	2U	10 A NZ 3-pin plug	Small office rack with one or two servers
USS003BR	3,000 VA / 3,000 W	Up to 96%	2,250 W	2U	IEC C20 (16 A), dedicated circuit	Larger office or school rack, PoE-heavy switching
USS006BR	6,000 VA / 6,000 W	Up to 95%	4,500 W	3U	Hardwired terminal block	Server room, multiple hosts and storage
USS010BR	10,000 VA / 10,000 W	Up to 95%	7,500 W	3U	Hardwired terminal block	Small data room, several racks on one UPS

External battery packs add rack height and are available for the 2 kVA to 10 kVA models; the 1 kVA runs on internal batteries only. Prefer floor-standing? The same range comes in tower versions.

Check 1: capacity

Take your measured peak load (see how to measure it) and pick the smallest model that keeps it at or below about 75% of rating. That leaves headroom for growth and inrush when equipment powers up.

If your load sits right on the line between two models, go up. Running at a lower percentage also improves runtime.

Check 2: runtime

Capacity tells you what the UPS can carry; it doesn't tell you for how long. Check the runtime at your load (see how much runtime you need). If the internal batteries fall short, add an external battery cabinet before considering a larger UPS: the 2U rack battery cabinet suits the 2 kVA and 3 kVA units, and the 3U rack battery cabinet suits the 6 kVA and 10 kVA units.

Find your UPS

Enter your measured peak load and the runtime you need. We'll show which models and battery options get you there.

Peak load (watts) Runtime needed (minutes)

Model	Load	Internal	+1 pack	+2 packs

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Check 3: the power supply to the rack

This is the check most often missed. At 230 V, a fully loaded 3 kVA UPS draws around 13 A, more than a standard 10 A socket is rated for. Larger units need a dedicated circuit, and 6 kVA and 10 kVA units need a hardwired supply installed by a licensed electrician to AS/NZS 3000.

Tip: while the electrician is on site, fit a maintenance bypass switch. It lets you service the UPS or replace its batteries without powering the rack down.

Check 4: rack space and distribution

Allow rack units for the UPS plus any battery packs, and put them at the bottom of the rack: they're the heaviest items. Then plan how power gets from the UPS to each device. A PDU sized to the UPS output, rather than a string of power boards, keeps the rack tidy and makes load easy to monitor. RackPower builds PDUs to order in Palmerston North if you need a specific outlet mix.

Not sure which fits?

Send us your equipment list and runtime target, and we'll recommend a UPS, battery and PDU combination.

Contact RackPower Call 0800 452 140

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