You Can Keep Your Old Transformer
Any older low-voltage transformer can be used to power both halogen and LED lights—even if they're mixed on the same circuit—as long as you have enough wattage to spare in your old transformer. If your transformer isn't big enough to handle the additional load, add a second transformer or upsize your existing one.
Avoid the Runway Effect
To prevent making your walkway look like an airport runway, stagger lights so they're not perfectly parallel.
Buy the Right Transformer
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.