How to size a portable air conditioner for your room

If you’ve ever bought an AC that runs all afternoon and still doesn’t cool the room — or one that short-cycles, leaves you clammy, and burns power for no reason — you bought the wrong size. Cooling capacity is measured in BTU/hr (British Thermal Units per hour), and matching the BTU to the room is the difference between a quiet, efficient unit and a noisy, expensive mistake.

The 20 BTU per sq ft rule (and why it’s only a starting point)

The US Department of Energy’s general rule of thumb is roughly 20 BTU per square foot of floor space for a standard room with 8′ ceilings, average insulation, moderate sun, and one or two occupants. That gives you:

• 150–250 sq ft: 5,000–6,000 BTU
• 250–350 sq ft: 7,000–8,000 BTU
• 350–450 sq ft: 9,000–10,000 BTU
• 450–550 sq ft: 11,000–12,000 BTU

This works for an enclosed bedroom in a temperate climate. For everything else, you adjust.

Adjust for these five real-world factors

1. Ceiling height. The rule assumes 8′. For 9′ ceilings add ~10% BTU; for 10′ add ~25%. A 300 sq ft room with 10′ ceilings needs closer to 10,000 BTU than 8,000.
2. Sun exposure. A heavily sunlit room (south-facing windows, west-facing afternoon sun, no blinds) needs ~10% more BTU. A shaded north-facing room needs ~10% less.
3. Number of regular occupants. Every additional person past two adds about 600 BTU to the load. A home office for one person and a family room used by four are not the same cooling problem.
4. Kitchen heat. If the AC has to cool a kitchen or an open-plan space with a kitchen attached, add 4,000 BTU to handle stove/oven heat output during cooking.
5. Insulation and climate. Older homes with single-pane windows and minimal insulation can need 15–25% more BTU. Hot southern climates (Phoenix, Houston, Miami) lean toward the high end of any range; northern climates lean toward the low end.

Why bigger is not better

Oversized AC units cool the air quickly but don’t run long enough to dehumidify. The result is a cold, clammy room with persistent humidity — uncomfortable, and it also encourages mold and condensation issues. Short-cycling (frequent on/off) wears the compressor faster and uses more electricity than running a properly-sized unit at steady state.

If you’re between two sizes after applying the adjustments above, pick the smaller one unless your climate is very hot or the room has heavy sun and kitchen load.

Portable AC specifics

Portable air conditioners exhaust hot air through a hose vented out a window. Compared to a window unit of the same nameplate BTU rating, a portable AC delivers roughly 20% less effective cooling because some heat from the exhaust hose radiates back into the room. The newer SACC (Seasonally Adjusted Cooling Capacity) standard reflects this; if a unit lists a SACC value, use it for sizing rather than the higher ASHRAE BTU.

Practical rule: for a portable AC, target a unit rated 1,000–2,000 BTU above what a window unit of the same room would need. A 300 sq ft bedroom that would take an 8,000 BTU window unit is more comfortable with a 10,000 BTU portable.

Sizing chart for portable AC

• Up to 200 sq ft (small bedroom, office): 8,000 BTU portable
• 200–350 sq ft (standard bedroom, living room): 10,000 BTU portable
• 350–550 sq ft (larger living area, open plan): 12,000+ BTU portable

What to do if you’re unsure

Measure the room (length × width = sq ft), note ceiling height, sun exposure, and number of occupants, then email contact@xhovn.com with that info. We’ll suggest a unit from our portable AC catalog. Sizing is the single biggest determinant of how happy you’ll be with an AC purchase — spend the five minutes to get it right.