Air conditioning units not only cool the air, but dry it as well. In the same way that air cools at night and forms a dew on the ground, the air that passes through an air conditioner forms a condensate on its coils. The condensate accumulates until it begins to drip into a tray or conduit for drainage. The amount that forms depends on the humidity of the indoor air, the rate at which it flows through the AC unit and the temperature of the air conditioner's coil.
Determine the temperature and relative humidity of the indoor air. You can measure temperature using a thermostat, and you can measure relative humidity with a humidistat, which may be a function of your thermostat or may be bought separately. Relative humidity tells you how much moisture is in the air as a percentage of the air's maximum moisture capacity at that temperature.
Look up the temperature setting of the air conditioner's supply stream. This is not the thermostat setting, but the temperature of the air after it passes through the coils and immediately before entering the room. This may be one of several distinct settings for commercial systems; if it is not explicitly set, measure the temperature within the duct work using a dry-bulb thermometer (dry-bulb thermometers are not affected by humidity levels).
Refer to a psychrometric chart to calculate the absolute humidity of the air at its supply temperature. A psychrometric chart is a graph that allows you to determine the relative humidity, absolute humidity, enthalpy and temperature of air, given two of these four metrics. If the air is producing condensate, it is at 100 per cent relative humidity and you can find the absolute humidity (the amount of moisture held, in weight, per volume of air) from the chart.
Find the difference in moisture content between a unit-volume of room temperature air and a unit-volume of AC supply air. If your humidistat gave the room's humidity in relative humidity, use a psychrometric chart to convert it to absolute humidity. Simply subtract the absolute humidity of AC supply air from the absolute humidity of room-temperature air to obtain the difference in water held per unit-volume.
Multiply the difference in moisture content per unit-volume by the air conditioner's total air flow rate. This will give you the amount of condensation produced per unit of time.
Because air is compressible, its volume will actually shrink slightly as it passes over the AC cooling coils. This diminution is proportional to the change in temperature, in Kelvins; a drop from 70 degrees to 40 degrees would compress the air by about 6 per cent. To correct for this phenomena, decrease the quantity of moisture in the supply-air stream (per unit volume) by 5 or 6 per cent before subtracting it from the quantity of moisture in room temperature air.