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A transformer is a small device that converts the high voltage power (120 V or 230 V, depending on your location) from your electric main to a lower voltage (usually 12 volts). There are many different varieties, so it is important to do your research. Some transformers have sensors that detect sunlight so that they automatically come on at dusk. However, many people have complained that the light sensors activate even when the sun goes behind a tree or cloud. So if you have many trees in your yard, you might want to consider getting a transformer where you manually set the time it will come on each evening. There are several things to consider when selecting a transformer.  Make sure to buy a transformer that can handle the lighting load you need it to. A transformer will be rated at a certain wattage; make sure that the wattage of your transformer exceeds the sum of the wattages of all your lights. Consider what kind of power switches the transformer has. Some transformers will have built-in timing switches to help you create a schedule for the lights to turn on and off. Others will have a  photo cell that can determine whether it is dark enough out to cycle the lights on. Make sure to get a transformer that fits your needs. Transformers are installed simply by plugging them into your outdoor outlet, though some larger ones must be mounted on the wall.

Summary:
Choose a transformer. Consider the features you need in a transformer. Install the transformer.