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Server Room Cooling and PUE Calculator

IT equipment turns almost all the power it uses into heat. Enter the watts to find the cooling in BTU/h and tons — or check a data center’s power usage effectiveness.

Method reviewed October 26, 2026. Version 1.0.0. How it works

Your inputs

Unit system

Measured draw from the UPS or PDU display is best. Power-supply nameplate ratings overstate the real draw.

Lighting, plus the losses of any UPS, transformer or PDU located in the room.

600 BTU/h is ENERGY STAR’s per-occupant addition for room AC sizing.

Usually small for an interior room. For a room with sunny outside walls or a roof, estimate it with the HVAC Load Calculator.

Result

Cooling load
19,367BTU/h
Cooling load
1.61tons

The room needs about 19,367 BTU/h (1.61 tons) of sensible cooling to remove the heat its equipment and occupants produce.

  • Equipment rooms produce almost no moisture, so this load is nearly all sensible heat. Choose cooling equipment by its sensible capacity, and plan for redundancy if the equipment cannot tolerate a cooling failure.
Cooling load
5.68kW

Breakdown

Electrical equipment and lighting
18,767BTU/h
People
600BTU/h
Walls, roof and windows
0BTU/h
Show the working
  1. Electrical heat = (IT + other) watts × 3.412
    (5,000 + 500) × 3.412 = 18,767 BTU/h
  2. People = count × allowance
    1 × 600 = 600 BTU/h
  3. Total and tons
    18,767 + 600 + 0 = 19,367 BTU/h ÷ 12,000 = 1.61 tons

Cooling load from watts

Every watt drawn by servers, switches, storage, lighting and any UPS or transformer inside the room becomes heat: 1 W = 3.412 BTU/h, and 12,000 BTU/h is one ton of cooling. Add an allowance for each person who works in the room and, for rooms with outside walls or a roof, the heat that comes through them.

The best input is measured power — the load shown on the UPS or rack PDUs. Power-supply nameplate ratings are maximums and usually overstate the real heat by a wide margin.

Choosing the cooling

An equipment room makes almost no moisture, so its load is nearly all sensible heat. Comfort air conditioners spend part of their capacity removing moisture, so compare the sensible capacity in the equipment data, not the nominal tons. Cooling should run all year, including in winter, and many rooms need a second unit so that a single failure does not shut the equipment down.

Power usage effectiveness

PUE is the facility’s total power divided by the power delivered to the IT equipment. A PUE of 1.5 means that for every kilowatt reaching the computers, another half kilowatt goes to cooling, fans, pumps, lighting and power-conversion losses. NREL (now the National Laboratory of the Rockies) reports that data centers average around 1.8 and that efficiency-focused facilities typically reach 1.2 or less. A single pair of readings gives an instantaneous PUE; a year of energy gives an annual PUE.

Worked example

These are the example values the calculator opens with, and the result the engine calculates for them.

Inputs

Calculate
Cooling load of the room
IT equipment power
5,000 W
Other electrical load in the room
500 W
People normally in the room
1
Heat allowance per person
600 BTU/h
Heat through walls, roof and windows (optional)
0 BTU/h

Result

Cooling load
19,367 BTU/h
Cooling load
1.61 tons
Cooling load
5.68 kW

The room needs about 19,367 BTU/h (1.61 tons) of sensible cooling to remove the heat its equipment and occupants produce.

Assumptions

  • Nearly all electrical power used in the room becomes heat in the room: 1 W = 3.412 BTU/h; 12,000 BTU/h = 1 ton.
  • PUE = total facility power ÷ power delivered to IT equipment.

Limitations

  • Uses the power you enter; if equipment grows, the cooling must grow with it.
  • Heat that leaves through ducted exhaust or a hot-aisle return to another space is not removed from this load automatically — account for it in the inputs.
  • PUE compares one facility over time; differences in what each site meters make comparisons between sites rough.

Frequently asked questions

How many BTU/h is a 10 kW rack?

10,000 W × 3.412 = 34,121 BTU/h, about 2.8 tons of cooling — if it really draws 10 kW. Measure the draw if you can.

Can I just use a comfort air conditioner?

Small closets sometimes do, but comfort units are designed for people, not for continuous year-round sensible loads. Check that the unit can run in cold weather and that its sensible capacity covers the load.

Can PUE be below 1?

No. Total facility power includes the IT power, so the ratio is at least 1. The tool refuses an IT figure larger than the total.

Sources

  1. NIST Guide to the SI (SP 811), Appendix B.9 — conversion factors — National Institute of Standards and Technology. 1 ton of refrigeration = 12,000 BTU/h; 1 BTU/h = 0.2930711 W; 1 atm = 101.325 kPa.
  2. Room Air Conditioners — sizing chart — ENERGY STAR (U.S. EPA / DOE). Floor-area sizing chart (5,000–34,000 BTU/h) and the shade, sun, occupancy and kitchen adjustments.
  3. High-Performance Computing Data Center Power Usage Effectiveness — National Laboratory of the Rockies (formerly NREL), U.S. DOE. PUE is the ratio of the total power used by a data center facility to the power delivered to its computing equipment; the overall average tends to be around 1.8, and data centers focusing on efficiency typically achieve 1.2 or less.

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