Once you have selected your garden lights, simply add up the total LED bulb wattage. If the total bulb wattage is under 60w, select the 60 watt transformer. For lighting wattages between 60w & 150w, you will require the 150w transformer. If the total is below 60w but you think you may wish to add more lights to the system in the future, select the 150w transformer. If the total wattage exceeds 150w, please call us for options. All transformers are weatherproof, and are supplied with UK plugs.