1. Calculate the power consumption by adding the wattage of all lights selected for the installation.
2. Take the total wattage and add 10% to compensate for fluctuations in wattage, or just jump up to the next sized transformer. Having a transformer with extra voltage will give you the power if additional lighting components are added in the future.
Example: You have 3 led lights that are each 6 watts. Find an appropriate transformer.
1. Add the wattage of all lights: 6 + 6 + 6 = 18 watts
2. Take the total wattage and add 10%. 18 watts + 1.8 watts (18 * .10) = 19.8 watts
So for this example, a 20 watt transformer would do the job or jump up to a 60 watt transformer to assure more lighting components can be added in the future.