A 'low-voltage' lighting system starts with a transformer plugged into a GFCI-protected receptacle. The transformer's job is to convert 120-volt household current to 12 volts before sending it through special outdoor cable to light your fixtures. Just a few years ago, you would have needed a 600- to 1,200-watt transformer to light a yard full of halogen lamps. But because LEDs use fewer watts than their halogen predecessors, smaller transformers—45 to 300 watts—are usually all that's needed.
So how big should you go? Just do the math! Simply add up the wattage for all the light fixtures you're planning to install, and use that number to pick the right transformer. Buy one slightly bigger than you need in case you decide to add fixtures in the future.
