When evaluating small air conditioners (portable or window units), prioritize the Cooling Capacity (BTU rating). For small rooms (150-250 sq. ft.), a 5,000 to 8,000 BTU unit is standard. Ensure the supplier uses High-Efficiency Compressors and check the Energy Efficiency Ratio (EER); a higher EER signifies lower operational costs. Additionally, verify the Noise Level (dB), aiming for under 50dB for residential use, and confirm the type of Refrigerant used, such as R-32 or R-410A, which are more environmentally friendly and compliant with modern standards.