To clean the condenser coils, use a cleaning chemical that with not erode the aluminum fins on the coil. HVAChelpdesk.com recommends NuBright. Anything cleaning agent that can break down dirt and cottonwood will work.  Spray the cleaning solution onto the condenser coils using a spray bottle.  DO NOT scrub the coil because you might bend the fins. Bending the fins has the same effect as a plugged coil, both reduce efficiency of the system. Allow the cleaning solution to sit for a minute to penetrate the dirt and grime. Then wash the coils very carefully with a garden hose. DO NOT over pressurize the water from the garden hose as this can cause the fins to bend. Use a cleaning solution even if can not visibly see dirt on the outside of the coil. The coil is a heating transfer surface and will be more efficient the cleaner it is. Therefore  cleaning with a cleaning solution is always recommended.  Some coils you can clean without taking the cover off the unit. To access other coils, you might have to remove the outside cover.