Size matters - choose your transformer with care!

A frequently asked question is “how many garden lights can I run from one transformer”? In fact, garden lighting transformers are available in a wide range of wattage ratings, so we need to look at the question in reverse: “what size of transformer do I need to power a group of garden lights”? This is simply a matter of multiplying the wattage and numbers of the lamps (bulbs) used in the garden light fittings (or “luminaire”) to be connected to the transformer - there is a help sheet on the subject which you can download.

In most cases we need to consider some spare capacity within the transformer rating so that we have scope to increase some lamp wattages as plants grow, or to allow for some flexibility in adding an extra spotlight or other garden light in future. This helps the garden lighting to “grow” with the garden. Allow at least 20% spare capacity in finalising the transformer choice. Transformer ratings are available in 50 or 100 watt steps, so it is merely a question of choosing the next step up from the total wattage you have calculated

Check that the total load is at least two-thirds of the transformer rating you propose to use; don't make the mistake of using a big transformer with only a small load (one or a few garden lights with low lamp wattages) as this can result in "overvoltage”, which reduces lamp life and increases maintenance of your garden lighting system.