Your air conditioner has an enemy, a nemesis to rival dirty air filters. Dirt, Debris, and overgrown plants are the outdoor equivalent of dirty filters. It is essential that your condenser gets as much undisturbed airflow as possible. That means no dirt on the coils, no bushes blocking off its sides, and no leaves piling up around it.
Why Is This a Big Deal?
Your air conditioner essentially collects heat from inside your building and then radiates it into the air outside the building. This has a few prerequisites: lots of air, lots of thermal-contact with the air, and ideally lots of airflow to replace the heated air with fresher, cooler air.
The condenser was designed to provide all of these things. There is a fan to suck in more air. There are thousands of tiny metal plates weaved back and forth inside the coils. These coils are made extremely conductive metals that will help radiate heat into the atmosphere.
Things go downhill fast when you start interfering with these features. Dirt is especially a vicious enemy here. The metal of the condenser coil is an amazing thermal conductor, readily blasting heat into the atmosphere. Dirt however, is not a good thermal conductor. Most dirt is going to act more like an insulator and reduce the amount of heat that flows from the condenser coil into the air around it. Less heat-flow means less capacity. Your 39,000 BTU system ends up hobbling along at 30,000 BTU or even worse.
The same happens when the airflow is obstructed. Bushes are especially a nuisance, able to grow up and cut off air flow around the condenser. The mass of leaves and branches is enough to restrict airflow. Every foot per minute is going to hurt your performance. Leaves are equally a disturbance, able to get sucked against the condenser and destroy your airflow. Less air means less heat can be removed and once more, your system is forced to lose performance.
The Bottom-Line Hitter
This comes back to bite you, hard and fast. The loss of performance causes the system to work harder and longer for the same amount of cooling. This increased operation causes increased wear on the system and increased electrical bills to operate it. As with all things, maintenance is key, especially for small things.
Keep the bushes trimmed back, keep the area around the condenser free of dirt and debris, and ensure that the internals of the condenser are cleaned a few times a year. Most HVAC professionals offer cleaning as part of scheduled maintenance services to keep your system running well.