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I'm looking to install a window AC in my son's bedroom, which is about 120 sq ft. The guides that I'm reading suggest a 5000 BTU unit due to the size. However we live in the high desert where the summer temperature gets > 100 degrees (F) at times during the summer. Also he has several reptile terrariums in his room that require lamps which also heat up his room. In this case would using a 6000 BTU AC unit be more appropriate?

My 180 sq ft. home office/spare bedroom is in the room next to his. I use a 6000 BTU unit and it chills this room no problem. So thinking of getting the a simular sized unit, but don't want to oversize if that's going to cause problems.

Thanks in advance for the advice.

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    An estimate of the terrarium lamps power needed.
    – fraxinus
    May 23, 2021 at 21:05
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    About 3 100 - 150 watt bulbs.
    – Frank
    May 23, 2021 at 21:26
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    Have you ever heard the term "solar gain"? Where are the rooms relative to the sun? (exposed west, tree-shaded, etc.) What color are the surfaces which face the sun?
    – Harper - Reinstate Ukraine
    May 23, 2021 at 21:29
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    @Frank 300W are ~1000BTU/h, 450W are ~1500 BTU/h . Whatever number you come to for the room, just add 1500 BTU/h for the lamps.
    – fraxinus
    May 23, 2021 at 21:34
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    @Harper-ReinstateMonica I was thinking the same thing, orientation of the room is an important consideration. Not only that, but quality (insulation ) of windows as well as the amount of insulation in the walls.
    – George Anderson
    May 23, 2021 at 21:48

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I would go larger, I have installed many heating and cooling systems and when the systems are running properly the AC will cycle more often, icing is more of a problem with very high humidity and or low air flow and this can happen even in the desert on a hot day if the filters are dirty reducing the air flow while the system is working at its maximum cooling capacity.

I find customers are happier being able to turn the thermostat up a little verses when the room is still hot with the AC at its lowest setting running full blast. An over sized unit may cost a little more because it may cycle the compressor on and off more often but this cost will be a small percentage of the annual cost to run and you have a better chance of keeping the room comfortable.

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