Rack Power & UPS Runtime Calculator

Calculate rack load, current per phase, UPS runtime and the heat you have to remove - the numbers a colocation order actually needs.

Calculator Cabling & Power Runs in your browser
Ah × V for the battery string.
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Result

Apparent power
3.37 kVA
3.2 kW real
Total current
14.65 A
Current per feed (normal)
7.32 A
14.65 A if one feed fails
UPS runtime at full load
34.5 minutes
Heat output
10,918 BTU/h
0.91 tons of cooling
Breaker size needed
32 A
at 80% continuous derating

Details

ItemValue
Real power (kW)3.2 kW
Apparent power (kVA)3.368 kVA
Power factor0.95
Annual energy at full load28,032 kWh
Runtime at 50% load69 minutes
Size each feed for full load
With A+B feeds each carries about 7.32 A normally, but each must be sized for the full 14.65 A because either can lose its partner. Sizing both at half load is the classic mistake that trips a breaker during maintenance.
Cooling
Every watt consumed becomes a watt of heat. A 3.2 kW rack needs roughly 11,000 BTU/h of cooling - cooling capacity, not power, is usually what limits rack density.

About Rack Power & UPS Runtime Calculator

Rack power planning needs four numbers: real load in kW, apparent load in kVA, current per feed, and the heat that has to be removed. Colocation is billed and constrained on all of them.

Why the redundant feed must carry everything

In an A+B arrangement each feed normally carries about half the load, and each must be rated for all of it. The moment one PDU, one UPS or one upstream breaker is taken out for maintenance, the survivor carries 100%. Sizing both at 50% is how a planned maintenance window becomes an outage. The same logic applies to the breaker: continuous loads should stay under 80% of its rating, so a 32 A circuit supports about 25 A of steady draw.

kW, kVA and the 80% rule

kW is real power consumed; kVA is what the supply and UPS must deliver, related by power factor. Modern IT power supplies achieve 0.95-0.99, so the gap is small - but UPS and generator ratings are given in kVA and sizing on kW alone undersizes them. Separately, continuous loads should not exceed 80% of breaker rating, so a 32 A circuit supports about 25.6 A of steady draw.

Common use cases

  • Specifying power for a colocation order.
  • Checking a rack build will not trip its breaker during a feed failure.
  • Estimating UPS runtime before adding equipment.

Edge cases and gotchas

  • Runtime falls faster than linearly as batteries age - a five-year-old string may deliver half its rated capacity.
  • Inrush at power-on can be several times steady-state draw; staggered startup avoids nuisance trips.

Frequently asked questions

How long should UPS runtime be?
Long enough to either ride out the typical outage or shut down cleanly. Five to ten minutes is common where a generator exists; without one, size for orderly shutdown of storage and virtualisation rather than for staying up.
How much cooling does a kW of IT load need?
About 3412 BTU/h per kW, since essentially all electrical energy becomes heat. Add margin for lighting, people and losses in the power chain.