There are three steps to calculate power supply for led lights
Low voltage lighting systems operate on 12 or 24 volts. The easiest way to start filtering out the right led power supply is to determine if the low voltage light fixture you need to power requires 12 or 24 volts. You should be able to find this information in the specifications on product. Also, know your power source input 120 or 220 v. Most power sources in the U.S. input 120 vac, but there are power supplies available for power sources that output 220 vac
Each power supply has a "maximum wattage load." The rule here is that the maximum wattage load of your transformer must be equal to or larger than the total wattage of all the light fixtures you are connecting to it. Let's say you have one light fixture with one 30-watt light bulb for the transformer mentioned above. That's fine. However, if you install a light fixture with three 30-watt light bulbs, you'll need a transformer with a maximum wattage load of at least 90 watts.
This simply means the power supply is encased in a metal case that provides storage space for terminal block connections you may need to make. It's ok to choose a transformer that's not enclosed, but you should probably store it in some sort of metal housing or a junction box.