Why offices need more cooling than homes
Residential air conditioner charts assume about 20 BTU per square foot. Offices are different: they are full of people (each adds roughly 400 – 450 BTU/h of heat), computers and monitors, printers, coffee machines and lighting, often with large windows. In a busy office these internal gains can account for half of the cooling load. CoolQuota starts from floor area, adjusts it for ceiling height, sun exposure and climate, then adds each internal heat source.
BTU/h = area × 20 × adjustments + people × 450 + equipment watts × 3.412 + lighting watts × 3.412
Units: BTU, tons and kW
One BTU per hour is a small amount of heat; 12,000 BTU/h equals one ton of cooling, and 3,412 BTU/h equals one kilowatt. Mini-splits are sold in 9,000, 12,000, 18,000, 24,000 and 36,000 BTU sizes; commercial systems in tons.
Typical office cooling loads
| Space | Typical load |
|---|
| Private office, 1 – 2 people (150 sq ft) | 5,000 – 7,000 BTU/h |
| Meeting room, 8 people (250 sq ft) | 10,000 – 12,000 BTU/h |
| Open office, 20 people (2,000 sq ft) | 60,000 – 70,000 BTU/h (5 – 6 tons) |
| Server closet, 3 kW of IT | 10,000 – 12,000 BTU/h, running 24/7 |
Choosing the type of air conditioner
Ductless mini-split
The most popular choice for single offices and small suites: quiet, efficient (SEER2 18 – 30), with inverter compressors that modulate output. One outdoor unit can feed several indoor heads in a multi-zone system.
Ceiling cassette
Four-way cassettes fit into suspended ceilings and distribute air evenly across open-plan areas and meeting rooms, typically 18,000 – 48,000 BTU/h.
Portable and window units
Useful as temporary or supplementary cooling, but noisier and less efficient. Single-hose portable units also pull warm air into the room; choose a dual-hose model if you must use one.
Server room cooling
Servers run 24/7, so give them a dedicated unit sized to the IT load (every 1 kW of IT equipment needs about 3,412 BTU/h) with low-ambient operation for winter. Don't rely on office comfort cooling that switches off at night.
Efficiency and running costs
SEER2 measures seasonal efficiency – higher is better. Moving from SEER2 13 to SEER2 20 cuts cooling electricity by about a third. Inverter units, programmable thermostats and occupancy sensors reduce costs further. Many utilities offer rebates for high-efficiency heat pumps, which also provide efficient heating in winter.
Avoid oversizing
A unit that is too big cools the air quickly and switches off before removing moisture, leaving the office cold and clammy, and wears out the compressor with short cycles. Size close to the calculated load and let an inverter unit handle peaks.
Frequently asked questions
How many BTU do I need for an office?
Start with about 20 BTU per square foot, then add roughly 450 BTU per person and 3.4 BTU per watt of equipment and lighting. A 600 sq ft office with 6 people and 6 computers needs around 20,000 – 24,000 BTU/h.
How many BTU per person?
Office workers add about 400 – 450 BTU per hour each from body heat.
What size AC for a server room?
Multiply the IT load in watts by 3.412. A 3 kW rack needs about 10,000 BTU/h, plus margin, from a unit rated for 24/7 operation.
Is a mini-split good for an office?
Yes. Ductless mini-splits are efficient, quiet and easy to install in existing offices, and multi-zone systems serve several rooms.
What happens if the AC is oversized?
It short-cycles, removes less humidity, uses more energy and wears faster. Choose the size closest to your load, preferably with an inverter compressor.
About CoolQuota
CoolQuota is an independent, free calculator for sizing air conditioners in commercial offices, meeting rooms and small server rooms. It uses simplified heat-load methods for preliminary sizing. We are not an HVAC contractor or manufacturer. A professional Manual N / heat-load calculation is recommended before installation.
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