Due to the reason I lack knowledge in air conditioning I asked a company that manufactures and sells a\cs to send a consultant to advise me on which air conditioner I should use in my room.

The suggestion he gave me is a ductless a\c with a BTU measure of 12,650.

I tried to learn about the quality of the suggestion and I found multiple sites that cite this table from energystar's site.

My room is 105 squared feet and is quite sunny so according to this chart a 5,500 btu a/c should be enough.

The suggestion by the consultant is very far from the estimation of this table and therefore I'm not sure what is the explanation. I'd like to understand which suggestion is correct to avoid the possibility of buying a highly non-matching a\c (either too strong or too weak).


Are there any other parameters, beside those that appear at energystar's site, that the consultant could take into account that result in this big difference?

For example I encountered a website that suggested that a hot climate could raise the required btu by 30%.

Is there any more information, about my room perhaps, that I can provide you to allow you to better answer my question?


I found this question. I concluded it doesn't contain enough information because:

  1. The calculator provided in one of the answers doesn't seem to fit very well to my single room situation (And when i tried it suggested even less than 5,500 btu).
  2. Some of the parameters listed there are also listed in energystar's site.
  3. Another parameter that was suggested was the climate. as I mentioned earlier this still doesn't cover the difference.


Information about the room

  • it's size is 105 sqr feet.
  • It has 2 windows of 8.7 sq feet each with double glass pane.
  • The climate is rather hot in summer (from 23 c at night to 32 c at day).
  • the door to my room is open during the day (which leads to the rest of the family house) and close during the night.
  • That is a cute table!! Definitely not real world. A 5500 btu ac is too small for a 105 sqf room! Are you going to be in this room? People give off heat. Are there electronics? They give off heat. You said sunny.... More heat. Yes, you can get a unit that is too big for an area.
    – Gunner
    Aug 19, 2019 at 18:18
  • Are you on the top floor? What is the facing of your outdoor wall(s)? Aug 19, 2019 at 20:25
  • why don't you ask the consultant about the method used to arrive at 12,650 btu?
    – jsotola
    Aug 19, 2019 at 21:42

4 Answers 4


Some key parameters:

  • Size
  • Peak Outdoor Temperature (i.e., needs to be sized based on the hottest days)
  • Insulation
  • Windows - size of windows and also quality (double or triple pane will do a LOT better at keeping cold in/heat out than single pane)
  • Frequency of opening of doors to the outside
  • Air Leakage - cracks, holes, etc.
  • Internal loads - how many people? computers or other equipment? A room full of servers generates a lot of heat.
  • size is 105 ft squared. peak outdoor temp is 32 celsius. I don't know about the insulation. My windows are 8.7 feet squared with double pane. the door to my room is open during the day and close at night. I don't know about any air leakages. Only I use the room and there's a single standard computer. Do you see anything that could explain the big difference? :) Aug 19, 2019 at 18:59
  • My best guess is that the recommendation was based on expected sales commission. Aug 19, 2019 at 19:34

Actually 105 soft is tiny, my living room is 16 x30 and it is a flat roof (no real insulation) 2 windows and a sliding glass door. I put a split 12k in there and it is a bit two big but I like to get it cool in summer and warm in the winter. My separate main unit 5 head is set up for each bedroom to have 9k inside then 12k in the dining room and 12k in the kitchen. this is large enough for the master and two big for the 144 sf small bedroom but as I said I like bigger and each room has its own control. The advantage of a heat pump would being able to efficiently heat in winter and cool in the summer. Window ac units usually use a heating element if they heat not efficient compared to a heat pump. Split systems are quiet at my age I cannot hear either outside unit and barely hear the inside units just a quiet fan when on turbo. A window unit is noisy and the best installs are not very well sealed and insulated, the proper sized window unit is usually cheaper by several hundred but the splits are more efficient and may make up that price difference over the units life. Last a window unit will not improve the value of your home but a built in unit will.

Without knowing climate zone you are in I can say a 4K - 6500 would be plenty large for that small of a room, that is really small for a split unit unless you go to multiple inside units a window model would be the route I would take unless noise is an issue.


Top issues are:

  • Solar gain
  • Solar gain
  • Solar gain
  • Size of the space, since it affects solar gain
  • equipment running in the unit
  • solar gain
  • ambient air temperature
  • solar gain
  • cooking
  • solar gain
  • number of occupants

Did I mention solar gain?

If you look at your unit from the sun's perspective, it is blasting every square meter of your exposed walls and roof with 1000 watts of energy or 3.6 megajoules per hour or 3000-ish BTU*. Per square metre. If you prefer square feet, 100 watts (360kj/hr, 300 BTU) per square foot, but that's from the sun's perspective, not floorplan.

If you are lucky, 50% of that gets reflected or radiated back out. And insulation will hopefully block some more, but keep in mind, insulation doesn't reject heat (it has no heat sink to dump it to), it only slows it down. Eventually, the full BTU rate will come through the insulation. Unless the sun sets first.

Ambient air temperature is a concern, but only so far as it warms the outside of the building (a pittance compared to solar gain) or leaks into the building.

Any equipment you have running in the space will, obviously, add its waste heat to the room, to the tune of 3 BTUs per watt expended. This is where that 850 watt gaming PC becomes your enemy.

That includes cooking, of course.

* BTU, as used in the HVAC trade, actually means BTU/hr in the scientific sense. This is trade slang.


Calculating the exact cooling requirement will be more difficult (costly) than upsizing the air conditioner. You're better to err on the large size.

The humidity load can be more than the cooling load. It takes much more energy to dehumidify a room than it does to lower the temperature. If your room is over a damp basement, or if there is little barrier to vapor passing through your walls, you'll need a much larger air conditioner.

If you have any warm, humid air entering the space, you'll again need a much larger conditioner.

Your walls can be insulated well, but still have many places where structural lumber conducts heat from outside into the room (this assumes a wood framed room).

Restating my original answer, it is less costly to oversize the cooling unit. The drawbacks to oversizing are:

  1. You may need a new branch electrical circuit to meet the power requirements.
  2. You may spend more money on the initial purchase.
  3. Potential temperature swings (too hot, followed by too cold). This becomes more likely if you wildly oversize the unit.

Oversizing has these advantages (assuming something reasonably oversized):

  1. You're more likely to have sufficient cooling capacity for edge case warm days.
  2. Your air conditioner will be on a lower percentage of time (quieter?)
  • Even oversized a 15 amp 120v unit could easily cool a small room like the op has. Dehumanizing is a byproduct of ac and doesn’t cost more to cool or swamp coolers would not be an option.
    – Ed Beal
    Aug 20, 2019 at 8:31
  • swamp coolers (aka evaporative coolers) lower the air temperature through evaporation of water. They make the room more humid. "Air conditioning" is a combination of air cooling and dehumidification. The combination of the two determine how comfortable a room feels.
    – ndemarco
    Aug 20, 2019 at 19:23
  • anything that changes the air is conditioning. A simple dehumidifier , humidifier, cooling loop or swamp cooler are all forms of conditioning. Dehumidification is a byproduct of most simple systems and in some cases a Humidifier is needed to keep the air properly conditioned.
    – Ed Beal
    Aug 20, 2019 at 21:54

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