The recommendation to let a new window air conditioner "settle" or stand upright for 24 hours before plugging it in is typically associated with units that have been shipped or transported. This advice is more commonly given for larger appliances like refrigerators and freezers, but it can also apply to window air conditioners.
The reason behind this recommendation is to allow the refrigerant and compressor oil inside the AC unit to settle properly. During transportation, the unit may be tilted or laid on its side, which can cause the refrigerant and oil to shift away from their intended positions. Allowing the unit to stand upright for 24 hours helps ensure that these fluids settle back into their correct locations before the unit is powered on.
By following this precautionary measure, you can reduce the risk of potential damage to the air conditioner and help it operate efficiently from the start. However, it's essential to refer to the manufacturer's specific guidelines and instructions for your particular model, as different units may have varying requirements. If the manufacturer's instructions recommend a settling period, it's best to follow their guidance to maintain the warranty and ensure the unit's optimal performance.