This guy is powering lights !!!
So who cares, when a voltage regulator consumes up to 2 Watts. The lights are NOT connected to the voltage regulator!!!!!!!
Using a 7809 or 7808 - you will have a voltage drop (including the integrated regulator of the Arduino) of 19 Volts (24 input minus 5 output).
If you use transistors or FETs instead of relais to control the lights, the current for the Arduino will still be below 100mA.
This means that you will see a maximum of less than 2 Watts of wasted power at the voltage regulators.. So what? Maybe you do not even have to use a heatsink.
Do not forget the context. Why would you want to improve a rocket that flies to the moon to arrive 10 seconds earlier. People who have problems usually seek for a solution that solves their problem - not a super super high technology solution that costs more and makes everything more complicated.
This guy just needs less than 100mA for his Arduino - and nothing else.
So all high tech solutions are just a waste of money and make things more complicated.
And by the way, switched power regulators have a very bad efficiency when the load is very low. When the load is 0.3 A or more, they do make sense - especially when you power your devices with batteries.
However, there are still situations, when an oldfashined linear voltage regulator makes sense.
The lights use 24 Volts directly as it seems - so please do not talk about lots of Amperes. All that is required in this thread is a certain voltage to operate an Arduino. This is not about powering lights that need lots of Amperes. All that is required is a voltage that will not destroy the Arduino.