You wouldn't need a transistor for a 16*2 LCD screen. The most power hungry bit is the backlight which needn't use more than can safely be supplied through a plain digital pin (with the right resistor). Buy the black on green ones and they don't need a backlight at all. Without the backlight the display and controller will use a milliamp or two total. The serial ones may use more. 'Sensors' could mean anything, most of my sensors use microamps so they wouldn't need a transistor either.