Power can be obtained from batteries, generated by spacecraft with built-in Solar Panels, or supplied from other spacecraft or modules.
Accessing the power view (shown by the lightning bolt insignia) by selecting the view tab in the top left corner of the screen. There, it is possible to turn the power on or off, turn on/off power switches between connected spacecraft/modules, and view the power being generated/used/supplied to other spacecraft.
To transfer power between spacecraft/modules, two spacecraft must be docked together, select the power view button, and toggle the switch. All spacecraft with enough solar panels will generate the power they need and will not need batteries to maneuver.
Docking and Un-Docking, Cargo transfer and Air and Fuel valve controls do not require Power supply to be performed.
Tutorial 9 teaches you about power management in the game.