So, if you want to connect ten landscape spotlights to a small transformer, and the lights utilize 2W / 4VA / 20 watts halogen equivalent light output power each, you would want a 100-watt transformer. (Example of calculations: 10 lights X 4 VA = 40 voltage amps).
How to Install Landscape Lighting – Planning Your Design & Sizing your Transformer
Frequently Asked Questions
How do you size a landscape light transformer?
Multiply the total wattage by 1.25 to adjust for loss due to cable distance. For the example, 240 x 1.25 = 300. The final number determines how large a transformer you need for your system. Choose a transformer that supplies no less than the required wattage and no more than 33 percent more.