The air conditioner in the apartment I rent has recently died. The owner wants to replace it with a new one, and (great relationship) we've discussed what to have installed.

Relevant information:

  • The unit being replaced is for the living room. A space of ~40sqm/440sqft total, ceiling is at 2.5m/8ft (so 100 m3, 3520 ft3 being cooled).
  • I live in a tropical climate: temperature is 30-40C/85-105F year-round, high humidity most of the year.
  • Surfaces through which I collect heat: single-pane glass balcony doors (9m2/100ft2) total, , load-bearing wall (25m2/270ft2) I think is not insulated but is facing adjacent unoccupied unit . Front door wall (7.5m2/80ft2) towards internal hallway. Other walls are internal to my apartment. Solid reinforced concrete ceiling and floor, to units above and below. No idea how thick, but I'd imagine chunky as I can never hear anything.
  • Used for 4 hours in the morning, 1 hour at lunch, 3 hours in the evening.

My questions, preparing for a talk to (in this country typically not very educated) the a/c engineer and owner:

  • What size unit should I be vying for? Current is 36000 BTU, which according to 'calculators' online seems high.
  • Given the use pattern, will an inverter use less energy?
  • What do you expect from an inverter?
    – Solar Mike
    Commented Dec 1, 2022 at 10:19
  • The inverter will save some money for you or the landlord, whoever pays the electric bills. It makes sense since you are not constantly in the unit. Personally I prefer an inverter in all the AC units in my rentals and have it in my own home.
    – RMDman
    Commented Dec 1, 2022 at 13:07
  • "Does an inverter make sense" is opinion based since you've given us nothing to calculate that on. In what way would it make sense? Financially? Would it work to cool the house? Who knows...
    – FreeMan
    Commented Dec 1, 2022 at 13:30
  • Agree that the 36,000 BTU value seems very high, even for your climate. Off the top of my head, I would think 12,000 - 15,000 BTUs would be plenty. A 36,000 BTU system is usually enough for a 1,500 - 2,000 sq ft house. Note that given your high humidity environment, you want the AC to run a lot in order to dehumidify the air.
    – SteveSh
    Commented Dec 1, 2022 at 13:42
  • 1
    Can you provide the area of the uninsulated walls and glass you collect heat through, and clarify if the ceiling and/or floor are also likely to be considered as exposed to the exterior (or apartments above/below that may not be cooled.) Is the wall construction masonry or some other material?
    – Ecnerwal
    Commented Dec 1, 2022 at 14:03

1 Answer 1


An inverter-drive (variable speed) AC is more efficient, and more effective at dehumidifying, as the speed will be adjusted to meet the load, rather than running at a fixed rate that's too much 99% of the time and shutting off periodically. That results in pulling out water all the time it's running, rather than periodically only when the compressor is running for a fixed-speed unit.

"Calculators online" tend to blandly assume some level of insulation, rather than getting enough inputs for a realistic estimate of heat load. For me, they come out too big as I'm well insulated. For you, with single-pane glass and lack of insulation, they will come out too small. Presumably the roof/ceiling is not insulated either?

Ballpark approximation assuming (on lack of actual data) a 5x8 meter room and ceiling exposed to air, floor not, with heat contributions from all the wall area (which I know isn't true, but I don't know what is true and it's an approximation), I get 17.5 °C below exterior for assumed masonry construction (typical for the tropics IME, until you clarify) and no insulation.

Which suggests that 36,000 BTU is likely larger than needed, but that depends just how hot it actually gets, for long enough that you'd not just put up with it for a rare very hot day where your living room would be cooler than outside but not as cool as you'd generally keep it. For some places, above 30°C might mean 30-38°C, for others it might mean 30-45°C and that would affect your sizing choice. Your profile indicates Bangkok, so probably more 30-38°C

  • 2
    Thanks for the comment. An hour after I posted, the condo owner made a decision to put in a 24,000 BTU wall unit to replace the 36,000 BTU existing unit. He did get an inverter model. The promised discussion never happened :-) Commented Dec 10, 2022 at 3:21

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