The water that drains from a central air conditioning unit is condensate that has accumulated on the cooling coils within the unit. Air conditioners work to cool the air by reducing indoor air temperature, but they also reduce air humidity, or the amount of water vapor in the air. This serves to make the temperature indoors feel cooler, and more comfortable. Central air conditioning units will drain more or less water depending on the humidity.
Condensation accumulates in your air conditioner as the unit cools the air. Indoor air is circulated across coils filled with cold freon. These coils accumulate condensation just as a bottle of cold water does in a hot room. The water then drips down in to a pan, and drains outside the home. This is a byproduct of the cooling system, and is not something to be concerned about.
On days where the humidity is higher, you may notice an increase in the amount of water your air conditioner is draining. This is completely normal, and is an artifact of the increased outside humidity increasing the indoor humidity and the air conditioning removing more water from the air. There is no water in the air conditioning system that can be leaking, so any water emanating from your central air conditioning unit is harmless condensate.
Overflowing Drain Pan
You will want to keep an eye on your drain to ensure that water is continuing to flow out freely. If you notice that water has not been draining over a period of a few humid days, it might be time to take a look at your unit and ensure the pan is not clogged. The pan within the unit that gathers the dripping condensate can become clogged if your air conditioner has been run for a period without a filter. Small amounts of debris or hair will clog the drain, and water will back up. In most modern units this flips a switch, turning the unit off, but in older installations it may back up and overflow.
Rough Estimate of Amount Per Day
Most modern air conditioners will produce between 5 and 9 gallons per day, varying based on unit performance, temperature setting, and outdoor humidity. Increasing water drainage from your air conditioning unit is not something to be concerned about. A dramatic decrease in water drainage without a corresponding decrease in outdoor humidity, however, may be indicative of a problem.