Air conditioner covers are rarely needed. An air conditioner is made to be open to the air.
Covers restrict air flow, holding moisture that affects electronic connections and creates oxidation and rust.
Leaves and debris in the air conditioner are the greatest
threat. Since air conditioners need air flow, they are not designed to keep out
leaves, seeds, rocks, and grass. A little debris is fine, but too much causes
problems. Decaying material on, or in, the unit creates corrosion.
And dirty condenser coils in the summer means your air conditioner needs more energy, which means your system runs longer, and wears out quicker.
Air conditioner covers may be useful at certain times, like during storms, with
wind, freezing rain, blowing sand or snow blizzard.
Even in the winter snow, it is better to lay a board over the top of the unit than it is to cover with a bag. Place plywood on top of the unit and weigh it down with rocks to keep it in place. The board prevents falling icicles and hail from damaging condenser coil, thermostat, refrigerant or casing.
That is because air conditioners are made to withstand weather and temperature extremes.
Also, small animals may shelter in the cover and chew wires and Freon lines for their nests.
And mold could grow in the damp air conditioner from lack of air flow. No cover is water proof, so choose a breathable cover with side vents allows for proper ventilation and airflow.
Finally, moisture is going to get in from the wind blowing in rain, humidity or
evaporation from the ground. You can’t stop it.
But covering the air condition is important for many people. Plastic is bad. Try commercial-grade vinyl or high-quality polyester. For these people, they cannot pull the cover all the way down to the ground. Halfway would be best.
The best covers are mesh air conditioner covers. They can be left on while the unit is running and protects from leaves and the elements.
Keeping the excess snow out of your air conditioner can help quite a bit. Cover your air conditioner with a tarp before a blizzard, and uncover it right after. It takes 15 minutes.
Remember to remove any cover or board on your air conditioner before use. Other wise, air flow is restricted, the unit won’t cool off, the over heating will shut down, and there may be damage.