This guide provides general and specific recommendations on application of step-up and step-down liquid-immersed and dry-type transformers in distributed photovoltaic (DPV) power generation systems for commercial, industrial, and utility systems. The guide focuses mainly on the inverter transformers of the DPV power generation systems that are connected to the inverters supplying ac voltage and current to the primary (LV) winding of the transformer. Some specifics attributed to the auxiliary power transformers in these systems are also discussed. Transformers covered in this guide comply with the relevant requirements defined in the IEEE Std C57.12.00 for liquid-immersed and IEEE Std C57.12.01 for dry-type transformers.