Basic Cooling Terminology

Fundamentals · 10 min read

Before you compare portable air conditioners, it helps to understand the terms on every spec sheet — starting with BTU and the efficiency ratings that affect your electric bill.

BTU (British Thermal Unit)

BTU measures cooling capacity — how much heat an air conditioner can remove from a room per hour. In the US, portable ACs are typically rated from 5,000 to 14,000 BTU.

Higher BTU means more cooling power, but bigger isn't always better. An oversized unit cools the air quickly without running long enough to remove humidity, leaving the room feeling clammy. An undersized unit runs constantly and never reaches your set temperature on hot days.

General sizing guide for standard 8 ft ceilings:

• Up to 150 sq ft: 5,000–8,000 BTU
• 150–250 sq ft: 8,000–10,000 BTU
• 250–350 sq ft: 10,000–12,000 BTU
• 350–450 sq ft: 12,000–14,000 BTU

Add 1,000–2,000 BTU for sunny rooms, kitchens, or spaces with high ceilings.

EER (Energy Efficiency Ratio)

EER tells you how efficiently a unit cools at a fixed outdoor temperature (usually 95°F). It's calculated as BTU output divided by watts of electricity consumed.

A higher EER means lower operating cost. Look for portable ACs with an EER of 8.5 or above. Premium models may reach 10 or higher. The yellow EnergyGuide label on many US appliances shows EER or related ratings.

SEER (Seasonal Energy Efficiency Ratio)

SEER is similar to EER but averaged across an entire cooling season at varying temperatures. SEER is more common on central air and mini-split systems; portable ACs more often list EER.

If you see SEER on a portable unit, the same rule applies: higher is more efficient. SEER 15+ is considered good for room air conditioners.

CFM (Cubic Feet per Minute)

CFM measures airflow — how many cubic feet of air the fan moves each minute. Higher CFM distributes cooled air across the room faster but can mean more fan noise.

For most bedrooms and offices, 150–250 CFM is typical on portable units. In open-plan spaces, higher airflow helps reach distant corners.

Decibels (dB) — Noise Level

Portable AC noise is measured in decibels (dB). Most units range from 50 dB (quiet conversation) to 60 dB (normal conversation) on high fan speed. Sleep-friendly models advertise 48 dB or lower on low speed.

Compare dB ratings at the same fan setting when shopping. Marketing terms like "whisper quiet" aren't standardized.

Single-Hose vs Dual-Hose

Single-hose portable ACs use one exhaust hose to vent hot air outdoors. They pull replacement air from gaps around doors and windows inside your home.

Dual-hose models have one hose for exhaust and one for intake from outside. They're generally more efficient in enclosed rooms because they don't create negative air pressure indoors.

Self-Evaporative and Condensate

Cooling removes humidity from the air, which collects as water (condensate). Self-evaporative units recycle most moisture through the exhaust hose. In humid climates, you may still need to empty a drain pan or connect a drain hose periodically.

ASHRAE vs DOE BTU Ratings

Since 2020, the US Department of Energy (DOE) requires a new testing standard for portable ACs. You may see both an older ASHRAE BTU rating and a lower DOE BTU (SACC) rating on the same box.

The DOE rating reflects real-world performance more accurately. When comparing models, use the DOE SACC figure if both are listed — it prevents over-sizing based on inflated numbers.

Bottom Line

BTU tells you if the unit is big enough. EER tells you how much it costs to run. CFM and dB affect comfort and noise. Understanding these basics makes every other buying decision — hose type, venting, brand — much easier.