Transformer calculation & selection is easy -- simply add up the total wattage of the garden lights in your installaton and choose your transformer based on the wattage needed, for example:
(7 Fixtures) X's (20 Watt) = 140 watts needed to power your fixtures, so it's best ot use a 300 Watt since a 150W transformer will not give you enough buffer. By code, transformers should be derated to 80% of their maximum wattage rating, so your new landscape lighting system functions safely & stays properly lit for many years ahead.