a basic question in electronics.
I have a DC device with input rating 12V--2A which means it consumes 12x2= 24 Watt DC
does it consume same amount of AC (by adapter) also while using AC to DC adapter?
But in real world:
* The AC adapter will have some power loss = heating.
* or in other words: the AC adapter will have a power efficiency of less than 1.
That is the maxum power that your load consumes. At low load, it would consume less anyways.
Your adapter has an efficiency curve against load demand. Your adapter efficiency is highest at peak load. As load decreases, the efficiency decreases.
I hope this is what you asked for.
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ADD:
Your adapter if properly designed will have a small power loss compared to the power that your load consumes.
So, just like Klaus has mentioned, you will lose some power during the AC_DC conversion. You need more than 24W at the AC_DC adapter input. The maximum power that you need at adapter input would now depend on the efficiency of your adapter.
where can i see this efficiency factor in adapter description?
take an example of smartphone's adopter. on adapter it is no where mentioned about its efficiency. however company claims that smartphone rewires 5 watts of DC.
Without knowing the exact specification of the power adapters nobody can tell you exactly.
A GUESS would be:
100 adapters at 6W each = 600W DC load
If the adapters use conventional transformers and linear regulators, assume an efficiency of 75% so the total power needed would be 600 * (100/75) = 800W.
If the adapters use switch mode supplies, assume an efficiency of 90% so the total power needed would be 600 * (100/90) = 667W.
As you can see, the adapter efficiency makes a big difference to the total. If you are lucky the adapter manufacturer will tell you how efficient it is but in many instances you don't even get to choose which adapter is supplied and you could even get a mix of different ones for the same purpose. Distributors know it has to supply 6W output but they will send you any adapter that meets or exceeds that rating.