Power transformers are used with a combination of batteries, solar panels, wind turbines, steam turbines, and other future power producing blocks/constructions. When connected to, for example, batteries and solar panels the transformer will provide power to an isolated area around the transformer( a large circle with the transformer in the middle, holding ctrl will show all powered areas in blue ). 

Connections are made possible using Power Isolators( small nob looking blocks ).  Place a power isolator on the transformer, place a power isolator on on of your batteries and one of your solar panels.  Right click the power isolators to create a blue connection line.  Right click a power isolator on your batteries or solar panels. Right click power isolator on power transformer again.  Right click whatever isolator you havnt connected yet( if you connected the panels, right click the battery isolator ).