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How many Btus is recommended for a room about 35 square meters? I have 2 units with 13,000 Btus each. I'm thinking it should be enough, but the room is just so-so cool. I can never say it's COLD in the room.

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I found this link, which is for a whole house: http://www.northernac.com/newcalc.htm

This one is for a single room. Assume the room direction is for the northern hemisphere. Reverse North and South if you're in the south. http://www.bhg.com/home-improvement/advice/measuring-materials/window-air-conditioner-size-calculator/>

From the comment by mike:

This calculator is a bit better in that regard, but does not take into account climate: thisoldhouse.com/toh/calculator – mike

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The BHG calculator is linear with respect to cubic feet, which is wrong. An A/C needs to be sized according to the thermal envelope, not the enveloped volume. For example, a 10'x10'x8' room contains 800 cubic feet, and 320 square feet of wall, where as a 12x12x8' room contains 1152 cubic feet (44% more) and 384 square feet of wall (20% more). Thus the 12x12 will loose only 20% more heat, while the calculator is saying it needs a 44% larger A/C unit. Moreover, the calculator does not take into account how much wall area is exterior. – mike Oct 23 '13 at 14:24
@mike Good points. – Chris Cudmore Oct 23 '13 at 14:27
This calculator is a bit better in that regard, but does not take into account regional climate: thisoldhouse.com/toh/calculator – mike Oct 23 '13 at 14:28
I wonder if local utility companies have A/C calculators, or if they advise on A/C needs during an (usually free) energy audit? – mike Oct 23 '13 at 14:35

Consumer Reports groups single-room A/Cs roughly into small/medium/large categories (100 to 300 square feet, 250 to 400 square feet, 350 to 650 square feet), and from what I've seen the BTU ratings and room sizes on A/C boxes in stores seem to follow the same general formula (probably they're all referencing the Energy Star calculations). Yes, one can quibble about floor space versus room surface area, but using the former as proxy for the latter seems to work well enough for most of the rooms in most of our houses.

However they note that Energy Star suggests you make some adjustments:

  • If the room is heavily shaded, reduce capacity by 10 percent.
  • If the room is very sunny, increase capacity by 10 percent.
  • If more than two people regularly occupy the room, add 600 BTUs for each additional person. (A human puts out roughly 100W of heat.)
  • If the unit is used in a kitchen, increase capacity by 4,000 BTUs.

Another factor: Energy efficiency. EER is the energy efficiency rating while under continuous operation; CEER is the rating with "energy saver" engaged (and automatic fan speed control, if the unit supports that). Energy saver modes try to turn the compressor and/or fan off when the target temperature has been hit, and turn the A/C back on only when the room has drifted a few degrees away from that temperature. The fluctuation is noticable and some find it annoying... but it does significantly reduce operating cost.

I went up a size on the unit I just purchased because the smaller one (a) was right on the edge of being able to handle the room size I needed without allowing for solar heating, and (b) had a CEER about 10% lower. Going up that step did increase the price from $120 to $180, but it was well within my budget either way. The biggest advantage of the small one would have been that it would fit in the narrow dormer window rather than the standard-width end-wall window.

(Now I need to decide whether it's time to replace the big one that covers most of the entertaining area. I try not to run that unless I must, so maybe I'll stick with the older and less-efficient for another year or two. Unless it makes the decision easy by dying.)

For what it's worth, GE's base recommendation (from the packaging) is:

  • 150 sq ft: 5000 BTU
  • 250: 6000
  • 350: 8000
  • 450: 10,000
  • 550: 12,000
  • 700: 14,000
  • 1000: 18,000
  • 1500: 24,000
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Also note that it's entirely reasonable to combine A/C and fans, which may let you get equivalent human comfort at slightly higher temperature and lower operating cost. – keshlam Jul 4 '14 at 6:59

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