Window air conditioners and portable a/c units have their respective advantages and disadvantages. There are some air conditioner we haven't tested on this list, like the the and the . Some window a/c units are an eyesore, but this design from Windmill is sleek and unassuming, all while working rather efficiently to cool down the home. Trevor RaabEnergy EfficiencyCEER, or Combined Energy-Efficiency Ratio, is the efficiency rating that applies to window air conditioners. You might see an EER (Energy Efficiency Ratio) number for your portable air conditioner, which measures the cooling efficiency of an air conditioner over an entire season.