When selecting a portable air compressor, prioritize CFM (Cubic Feet per Minute) and PSI (Pounds per Square Inch) ratings. For heavy-duty pneumatic tools, ensure the CFM at 90 PSI meets the tool's requirements. For automotive or emergency use, a maximum PSI of 150 is standard. Additionally, consider the Power Source: 12V DC for vehicle use, 110V/220V AC for workshops, or Lithium-ion battery-powered models for maximum portability. Always verify the Duty Cycle; a 50% duty cycle means the unit needs equal rest time to prevent overheating during continuous operation.