A capacitor is a clever device composed of two metal conductors separated by an insulating material. On an air conditioner, they often look like large, cylindrical batteries with two or three posts sticking out of the top. The main job of capacitors is to store electrons to provide start-up energy for your air conditioner, they build up a charge when electricity runs through them by swapping electrons between the two conductive plates inside. Because of their capacitors, air conditioners do a little magic trick every time they start up. Your A/C unit actually requires a great deal more start-up energy than is available through your home’s wiring, so the capacitor is added to the circuit to give an electrical jumpstart at the same moment that your air conditioner is drawing power from the electrical grid. Together, these two electrical sources provide the right amount of juice to your air conditioner. The capacitor’s job is over until an air conditioning cycle is complete. Then the compressor must start up again. Your air conditioner may actually contain several different capacitors, including the compressor motor run capacitor, the outside fan motor run capacitor, the indoor fan motor run capacitor and the start capacitor. The most common capacitor to fail is the compressor motor run capacitor. It has a big job, and an even bigger footprint in your air conditioner. It’s actually a dual capacitor, with three terminals instead of just two.