Transformers are required for low voltage pumps and all submersible lights. Most low-voltage pumps come with transformers as 'kits'. If there is a requirement for a transformer, select it according to 1. output voltage, and 2. power as expressed in VoltAmps (VA) - equivalent to Watts. The other important consideration is cable length - long lengths of cable may result in voltage drop causing damage to the pump. Please ask for advice if unsure. All transformers or plug packs listed are AC not DC.