My current project will be sitting next to a 12VDC power line. It seems a waste no to tap it to power into the Arduino board too. Now I read everywhere that with 12V input quite a lot power gets dissipated by the voltage regulator. Wouldn't it then a wise move to use the board 's own PWM capabilities to throttle the input voltage?
Nother point is whether it is sensible at all when the board is using little current (in this case less than 5mA over the pins for 0.1 sec every few seconds, plus the board in deep sleep mode).