I hope this is not a duplicate, because I am sure other people have the same question...
And could someone explain the reasons why?
N.B. From the 'scienceabc' website linked to below:
Despite being so celebrated, DC exhibited a major problem — transmitting it over longer distances was really difficult. The wires lost power and had to be rejuvenated using additional circuitry.
At what distance(s), approximately, are high-voltage DC (HVDC) power lines \$\boxed{\text{more efficient}}\$ than AC lines?
When the OP writes \$\boxed{\text{more efficient}}\$, I take it to mean efficient in terms of minimizing power loss
So, basically at any distance DC is more power efficient than AC.
DC is a more efficient transporter of power than AC due to skin-effect increasing AC power line losses and reactance causing AC regulation problems at the load. However, if you are looking for the reason why we normally transport power using AC then, there are a whole number of other things (unrelated to power efficiency) to consider.
AC is more efficiently \$\boxed{\text{generated}}\$ than DC and, AC transportation can make use of transformers to change voltages to suit the transport section.
This Link explains in a basic way the differences of AC versus DC in terms of efficency and why one is favoured over the other.
AC can be stepped-up to a higher voltage, to overcome the resistance. And then again stepped-down to a lower voltage, but with a higher current. DC is better in preserving power. As AC loses power for a moment when switching from positive to negative.
This second link shows that for a big portion of the day DC would be more benficial in residential areas. So as @Andy aka answered, AC can be generated more efficiently, while DC will be more efficient in providing the needed power.
