Arduino Power supply -from 24V to 5V how much consumption ?

Hi following my later problems with arduino i decided to get some external power instead of the usb connection…
My arduino is sensing my 24v battery bank with a divider and i want to use the already 24v inside the box to power it …
I will need some dc-dc power down … searching my box of “garbage”… i found some of this based on the LM2576T

i know the LM2576T can handle 40V input so for my 24(max29V) system is ok, i believe…
now my question is using the lm2576T with this voltage drop from 24 to 5 what will be my power consumption (arduino should be under 0,5amp) 0,5amp + x loss… how to calculate this loss ?
i want to know how much will this drain my batts…

Regards.
Ps: i have other with LM2596<— is this one better ?

There is graph on page 8 in the datasheet showing efficiency for input volts Vs output volts, and that at 24vdc in and 5vdc out, the device is around 78% efficient. I'll leave the math to you, but that should be all the input variables needed to determine that.

http://www.national.com/ds/LM/LM2576.pdf

So measure

Hi & thanks for the answer, i must apologise for my ignorance but i'm not getting .. trying to understand.... if it was 100% efficiency would mean that i was only using the power arduino was using (0,5amp at 5 volts=~2,5 watts ), right ? 78% efficiency means that i would use more (100-78=22%) than the 2,5w 2,5w/0,78 =~3.02w , correct ?

^^^^^^^^^^^^^^^^^^^^^^ This make sense ??

thanks

This make sense ??

Well 78% of 3.02 (Watts input power) = 2.35 watts of output power, which does not match your 2.5 watts of power consumed by the arduino. So your math with percentages needs work, however you are close and should get the point. By the way if you were using a typical 7805 regulator at 24vdc input, it would be wasting around 9.5 watts in heat dissipation supplying the arduino with 500ma of current. Calc the efficiency of that to gain an appreciation for switching voltage regulators Vs Linear voltage regulators.

Lefty