Thanks to the wide variety of green outdoor lighting products, garden lighting can be affordable and practical. All low voltage lighting products consume less energy, so choosing the correct wattage has more to do with the results you are expecting to create on your garden than with the energy consumption of the bulb.
For example, on path area lighting, you may want to choose your bulbs according to the area you want to cover. The bigger the circumference, the brighter it should be. The illustration below provides an example of the diameter and bulb power.
When creating a focal point in your garden try to envision how far the object you want to shed light on will be from the viewer. An object located far away from the main view point will require a brighter light than one that is closer. The illustration below is an example of a directional light spotting a tree and the appropriate wattage for the different view points.
Try around different options to see what works best for your garden. If you need some ideas on how to simulate different lighting effects take a look at this video How to Check the Layout of Landscape Lighting in Your Yard by Tricia Craven Worley on Today’s Home Owner.