I found this answer to a related question. The part of the answer that's confusing me is: 


  Transmitting DC power over a long distance is inefficient. Thus AC supply is a far more efficient to transmit power.


According to Siemens it's quite the opposite: 


  Whenever power has to be transmitted over long distances, DC transmission is the most economical solution compared to high-voltage AC.


Also, from Wikipedia 


  HVDC transmission losses are quoted as less than 3% per 1,000 km, which are 30 to 40% less than with AC lines, at the same voltage levels.


Is the posted answer correct?

- - EDIT - -

Chris H made a very important observation (see his comment below): The context of the post I mentioned was of low voltage whereas I was blindly thinking of high voltage. Indeed I learnt loads by the answers and comments. Thanks.  

It is more efficient to transmit DC using about the same infrastructure.  This is because of several effects:

Skin effect experienced with AC.  There is no skin effect with DC.

Higher voltage allowed with DC for the same transmission lines.  The lines have to withstand the peak voltage.  With AC, that is 1.4 times higher than the RMS.  With DC, the RMS and peak voltages are the same.  However, the power transmitted is the current times the RMS, not peak, voltage.

No radiation loss with DC.  Long transmission lines act as antennas and do radiate some power.  That can only happen with AC.

No induction losses.  The changing magnetic field around a wire carrying AC current causes induced voltage and current in nearby conductors.  In effect, the transmission line is the primary of a transformer, and conductors near it are secondaries.  With DC current, the magnetic field doesn't change and therefore doesn't transfer power.



Another advantage of DC is that it doesn't require synchronization between grids.  Two AC grids need to be phase-synced to be connected together.  This gets tricky when the distances are large enough to be significant fractions of a cycle.

The flip side is that AC is easier to convert between voltages.  Converting DC back to AC to dump it onto the local grid at the receiving end is not a trivial process.  It takes a large plant to do this, which means significant expense.  That expense is only worth it if the transmission distance is long enough so that the efficiency savings outweigh the cost of the DC-AC conversion plant over its lifetime.

Here is a example of what it takes to convert high voltage DC back to AC:



DC power from large dams in Quebec enters at top right.  This plant converts that to AC and dumps the power onto a large regional AC transmission line in Ayer Massachusetts at 42.5702N 71.5242W.

The expense of building and operating this plant is worth it due to the significant power savings of transmitting DC instead of AC.  Synchronization was also a factor in using DC.
