Portable AC BTU Calculator

The short answer

Size a portable air conditioner by its SACC rating, not the bigger ASHRAE number that’s often printed next to it. Work out the cooling your room needs from ENERGY STAR’s room chart, then look for a portable unit whose SACC rating is at least that much. A studio apartment that needs 8,000 BTU needs a portable AC rated at least 8,000 BTU SACC.

The calculator works out the room’s needs, with ENERGY STAR’s adjustments for sun, people and kitchens, and tells you the SACC rating to look for.

How to use the calculator

1. Measure the room wall to wall at floor level, and measure the ceiling height. The chart assumes 8 ft.
2. Pick the sun the room gets and count the people who regularly use it at the same time.
3. Tick the kitchen box if the unit will cool a kitchen. Cooking adds 4,000 BTU to what the room needs.
4. Pick the hose type if you know it. It changes the advice, not the number, because the SACC rating already accounts for it.
5. Compare the result with the SACC rating on the box or spec sheet. It may also be called the DOE rating.

SACC ratings by room size

The SACC rating to look for in a room with average sun and up to 2 people, from ENERGY STAR’s room chart:

Add 10% for a very sunny room, 600 BTU for each extra person and 4,000 BTU for a kitchen, or let the calculator do it.

SACC vs ASHRAE: which number to trust

Portable air conditioners often list two cooling ratings, and they can be far apart.

• ASHRAE is the older rating. It measures the cooling the unit produces at its vents.
• SACC (seasonally adjusted cooling capacity) is the rating the US Department of Energy requires. Its test subtracts the heat that leaks back into the room from the hot exhaust hose and from the warm outside air the unit pulls in to replace the air it blows out. It also blends results at two outdoor temperatures.

The Department of Energy found that the real cooling of portable units is “generally significantly lower” than the older test suggested, mostly because of that outside air. It also says there’s no fixed way to convert one rating into the other, because airflow and heat losses vary between models. As an example of the gap, LG notes that the DOE rating can be up to 42% lower than the ASHRAE rating for the same product.

So when two portable units both say “8,000 BTU” in big type, check which rating that is. Only the SACC number fits the chart.

How we calculate

1. Room needs. We use ENERGY STAR’s room air conditioner chart and adjustments, the same way as our window AC BTU calculator: the chart capacity for your floor area, 10% less for heavy shade or 10% more for strong sun, 600 BTU for each person beyond 2 and 4,000 BTU for a kitchen.
2. Tall ceilings. Above 8 ft, we scale the floor area by your ceiling height divided by 8 ft. That’s our own step; ENERGY STAR only says you may want a bigger unit.
3. Matching the rating. ENERGY STAR’s chart is written for room air conditioners, and ENERGY STAR doesn’t publish one for portable units. We match it against the SACC rating because SACC is the Department of Energy’s measure of the cooling a portable unit actually delivers to the room. That pairing is our choice, and we say so here.

We don’t round up to store sizes for portable units, because we couldn’t find a reliable list of the SACC ratings on sale. Pick a unit at or above the number the calculator gives.

Worked examples

Example: A studio apartment

What we know

• Room: 20 ft by 15 ft (300 sq ft), 8 ft ceiling
• Sun: Average
• People: 2

How it works out

1. Chart: 300 sq ft is in the 300 to 350 sq ft row, which calls for 8,000 BTU.
2. Compare that with the SACC rating on the box, not the bigger ASHRAE number.

A portable AC with a SACC rating of at least 8,000 BTU.

Example: A sunny bedroom

What we know

• Room: 14 ft by 12 ft (168 sq ft), 8 ft ceiling
• Sun: Very sunny
• People: 2

How it works out

1. Chart: 168 sq ft is in the 150 to 250 sq ft row, which calls for 6,000 BTU.
2. Very sunny: + 10% of 6,000 BTU = 600 BTU.
3. Needed: 6,600 BTU.
4. Compare that with the SACC rating on the box, not the bigger ASHRAE number.

A portable AC with a SACC rating of at least 6,600 BTU.

Example: A room with a high ceiling

What we know

• Room: 15 ft by 12 ft (180 sq ft), 10 ft ceiling
• Sun: Average
• People: 2

How it works out

1. Ceiling: the chart assumes an 8 ft ceiling, so we count 180 sq ft × 10 ft ÷ 8 ft = 225 sq ft.
2. Chart: 225 sq ft is in the 150 to 250 sq ft row, which calls for 6,000 BTU.
3. Compare that with the SACC rating on the box, not the bigger ASHRAE number.

A portable AC with a SACC rating of at least 6,000 BTU.

What it costs to run

Portable air conditioners made since Oct 1, 2022 carry a yellow EnergyGuide label with an estimated yearly energy cost. Use it to compare models of the same size. The FTC’s label assumes 13¢ per kWh and 8 hours a day for 3 months, so at the US average of 18.19¢ per kWh (January to August 2026, from the U.S. Energy Information Administration), expect about 40% more than the label says, and more again if your utility charges above the average.

Single hose or dual hose?

A single-hose unit blows hot air out through its hose, and the room replaces that air with warm air from outside, through gaps around doors and windows. The Department of Energy notes that single-hose units “typically generate higher rates of infiltration air” than comparable dual-hose units, and that this affects how much they cool. A dual-hose unit draws outside air through a second hose to cool its parts, though the Department of Energy notes it typically still takes a portion of that air from the room.

The SACC test measures that effect, so you don’t need to adjust for it yourself. Compare SACC numbers, whichever type you’re looking at.

Renter tips for portable units

• Venting. The exhaust hose has to vent outside, usually through a window kit that fills the gap around the hose.
• Fire escapes. Don’t vent through a window you’d need as a fire escape. New York City’s Department of Buildings gives the same rule for air conditioners in general.
• Your lease. Nothing is fixed to the building, but some standard leases, such as the Real Estate Board of New York’s form, require the owner’s written consent for any air conditioning unit, so check yours. Oregon and Washington limit when a landlord can stop a tenant from using a portable cooling device.

Check the spec sheet for the SACC number before you compare prices. Two units with the same big number on the front can cool very differently.