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How to Size a UPS

Oct 21
5 min read

Updated: Oct 22

Eaton frames UPS sizing as two questions: how much power the connected equipment needs (capacity), and how long the UPS must carry it during an outage (runtime) (Eaton UPS sizing guide). Schneider Electric (APC) adds the plug and circuit the UPS connects to, and the UPS topology (Schneider FAQ000268376). This guide walks through each one using published manufacturer figures.

New to how UPS systems work? Start with How UPS Systems Keep Critical Loads Online, then come back here.

Step 1: Add up the load in watts

List all the equipment that will plug into the UPS. Eaton recommends getting each device's wattage from the manufacturer's documentation, or from the nameplate if there's no documentation. If a device lists only amps, multiply amps by the AC voltage to estimate watts (Eaton, Top 12 Questions).

Add the watts together. That total is your load.

Eaton also advises never plugging a laser printer into a UPS's battery outlets, because it can make the unit transfer to battery repeatedly. A surge-only outlet is fine for one (Eaton UPS buying guide).

Step 2: Understand VA versus watts

Every UPS is rated in both volt-amperes (VA) and watts (W). Eaton calls watts "the key rating because it represents the amount of real power available" (Eaton UPS buying guide). The watt rating per VA (the power factor) varies from one UPS to the next. Here are three ratings Xtreme Power Conversion publishes:

UPS

VA rating

Watt rating

Watts per VA

S71-1500

1500 VA

900 W

0.6

P80-1500

1500 VA

1350 W

0.9

P91-1500

1500 VA

1450 W

about 0.97

All three are "1500 VA" units, yet they're rated for 900, 1350 and 1450 watts. Compare your load against the watt rating, and check that the VA rating isn't exceeded either.

Step 3: Leave headroom

Schneider Electric recommends against loading a UPS above 80%, to allow headroom for inrush current when equipment starts up (Schneider FA156513). It also recommends sizing so the connected load is "preferably around 80% of the UPS' maximum capacity", noting that running near full capacity lowers efficiency, adds heat and reduces runtime (Schneider FAQ000268376). Eaton gives the same 80% limit (Eaton, Top 12 Questions).

To find the minimum watt rating that keeps your load at or below 80%, divide your load by 0.8.

Step 4: Decide how much runtime you need

Runtime is how long the UPS can carry your load on battery. It doesn't scale evenly with load. Xtreme's published table for the P91-1500 lists about 10 minutes at 50% load and about 3 minutes at 100% load on its internal batteries (Xtreme P91 flyer).

Eaton describes the battery system's job as supporting the load long enough to restore power or to shut systems down safely (Eaton UPS sizing guide). If you need more runtime than the internal batteries give, choose a model that accepts external battery packs. In Xtreme's P91 table, adding one BP36 pack to a P91-1500 raises full-load runtime from about 3 to about 17 minutes. The T91 accepts T91-BP36A and T91-BP72A packs, and the J90 flyer lists extended runtime for its 1.5, 2 and 3 kVA models with the J90-EBP48 pack.

Read the manufacturer's runtime table at your load, not just the full-load figure. Xtreme notes that its runtimes "will vary based on battery condition, age, cycles, and ambient temperature". For background on capacity and discharge rate, see Battery Basics: Voltage, Capacity and C-Rate.

Step 5: Match the plug, the outlets and the space

  • Plug. Schneider advises checking that the UPS input plug matches the voltage and amperage rating of the outlet (Schneider FAQ000268376). In the Xtreme P91 line, the 1000 VA (2U) and 1500 VA models use a 5-15P plug, the 2000 VA a 5-20P, and the 3000 VA an L5-30P (Xtreme P91 flyer).

  • Outlets. Count your devices and check the receptacles on the UPS. The P91-1500, for example, has (8) NEMA 5-15R.

  • Form factor. Xtreme lists the J90-1.5kLiD as a 1U design. The P80 and P91 support tower, wall, 2-post and 4-post rack mounting. The T91 is an online tower UPS. The J60C-600 is a 1U UPS 8.5 in deep for short-depth racks (J90 flyer, P80 flyer, T91 flyer, J60C flyer).

Step 6: Choose the topology

Eaton describes line-interactive UPS systems as using automatic voltage regulation (AVR) to correct abnormal voltages without switching to battery. Online UPS systems instead continuously convert incoming AC to DC and back to AC (Eaton, Top 12 Questions).

  • Line-interactive: Xtreme lists AVR on the S71. Its P80 flyer says the line-interactive P80 corrects voltage using transformer taps rather than the battery.

  • Online double conversion: the P91, T91 and J90.

For small office and home office use, Eaton says standby and line-interactive UPS systems properly support and protect the equipment (Eaton UPS buying guide); see also Home Office UPS Benefits. Schneider recommends double conversion online when the load has redundant power supplies or needs an automatic transfer switch (Schneider FAQ000268376).

Worked example (illustrative): a small office network rack

Example only. These loads are made-up numbers to show the method. They are not measurements or a recommendation for any real site.

Device

Watts (example)

Firewall

60 W

24-port PoE switch

150 W

NAS storage

120 W

Small server

250 W

Total load

580 W

  1. Headroom: 580 W ÷ 0.8 = 725 W. To stay at or below 80% load, the UPS needs a watt rating of at least 725 W.

  2. Watts, not VA: the S71-1000 is rated 1000 VA but only 600 W, so it falls short. The P91-1000 is rated 1000 VA / 1000 W and clears 725 W.

  3. Runtime: 580 W is 58% of the P91-1000's 1000 W rating. Xtreme's table lists 10 minutes at 50% load and 3 minutes at 100%, so runtime at 580 W falls between those two figures. For more runtime at about this load, the published tables show:

    • P80-1500 (1350 W): 18 minutes at 50% load (675 W). It accepts P90-BP48 battery packs.

    • P91-1.5kLi (1350 W, lithium): 22 minutes at 540 W.

    • P91-1500 with one BP36 pack: 42 minutes at 50% load and 17 minutes at 100%.

  4. Plug: Xtreme lists a 6 ft 5-15P line cord for the P91-1000 (2U), P91-1500, P80-1500 and P91-1.5kLi.

In this example, the P91-1000 meets the 80% guideline, and the larger units or a battery pack add runtime, according to Xtreme's published tables.

Quick checklist

  • Total watts of the equipment, from manufacturer documentation or the nameplate

  • UPS watt rating of at least your load ÷ 0.8 (the 80% guideline), with the VA rating not exceeded

  • Runtime read from the manufacturer's table at your load

  • External battery packs if you need more runtime than the internal batteries give

  • Plug and circuit, outlet count and mounting confirmed against the spec sheet

  • Topology chosen for your equipment and site

Need help picking a model?

Send us your equipment list and the runtime you need, and we'll help you match a model and battery option.

Request current price and availability: use the request form or email office@luxena.net.

Sources

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