Look at one that has two pipes, an intake and an exhaust. The ones with only an exhaust are actually causing indoor air to go outside, and pulling outside air in. Since the compressor and condenser are in the unit, they need to be cooled so the resulting hot air needs to go outside, but it's very inefficient if it's using indoor air to cool it as this air needs to be replaced. That only occured to me after I bought my 1 ton unit. I modified it so it takes outside air to cool the compressor and increased it's efficiency like 4x. Some are designed like this, so that's your best bet if buying one.
I know this is a bit of a resurrection but if your still looking for a portable air conditioner when things start heating up this summer take a look at consumersearch.com. They generally are a decent source of unbiased reviews because they simply aggregate reviews from websites. Unfortunatly they don't have any two hose units listed in their reviews for whatever reason. Two hose units are indeed more efficient and I'd recommend one if you can find it in your price range. I'm assuming window air conditioners are out of the question because of some sort of ordinance or restriction? Window units also tend to be a bit more efficient than portable air conditioners because the compressor sits outside of the house and heat from that stays outside.
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