What is the ENERGY STAR sizing rule?

ENERGY STAR recommends roughly 20 BTU/h of cooling per square foot of floor area for a room air conditioner. The base capacity is then adjusted up for sunny rooms, extra occupants and kitchens, and down for heavily shaded rooms, to match the room's real cooling load.

What adjustments does it apply?

It reduces capacity by 10 percent for a heavily shaded room and raises it by 10 percent for a very sunny one. It adds 600 BTU/h for each regular occupant beyond two people, and adds 4,000 BTU/h if the room is a kitchen, where appliances add significant heat.

Why not just buy the biggest unit I can?

An oversized air conditioner cools the air fast but cycles off before it removes humidity, leaving the room cold and clammy and wasting energy. Matching capacity to the room keeps run times long enough to dehumidify and run efficiently.

Does this work for mini-splits and portables?

Yes. The BTU figure is a cooling capacity that applies to window units, ductless mini-splits and portable units alike. Portable units lose some effective capacity through the exhaust hose, so round up to the next size when choosing a portable.

Does ceiling height matter?

The 20 BTU/h per square foot rule assumes a standard 8-foot ceiling. For notably higher ceilings, increase the area input proportionally or add roughly 10 percent per extra two feet of height, since you are cooling more air volume.

Room BTU Calculator — Gera Tools

Picking the right size air conditioner is the difference between a comfortable, efficient room and a cold, clammy one that short-cycles. This calculator uses the ENERGY STAR room air conditioner sizing method to turn your room’s size and conditions into a recommended cooling capacity in BTU per hour.

How it works

The ENERGY STAR rule starts from floor area and adjusts for the things that change a room’s real cooling load:

base       = floor area (ft²) × 20 BTU/h
sun         × 1.10 if very sunny, × 0.90 if heavily shaded
occupants  + 600 BTU/h for each regular occupant beyond two
kitchen    + 4,000 BTU/h if the room is a kitchen
recommended = base × sun + occupant add + kitchen add

Twenty BTU/h per square foot assumes a standard eight-foot ceiling. The adjustments capture solar gain, body heat from extra people, and appliance heat in kitchens, which all raise the load beyond the bare floor area.

Example and tips

A sunny 300-square-foot living room with three regular occupants needs about 300 × 20 × 1.10 = 6,600 BTU/h, plus 600 for the third person, for roughly 7,200 BTU/h — comfortably handled by an 8,000 BTU unit. For a kitchen, add the 4,000 BTU/h kitchen allowance. If you are buying a portable unit, round up one size because the exhaust hose costs you some effective capacity, and increase the area input for ceilings taller than eight feet.