[Symbiote]

Isn't that the case with a grid of any size?

The British grid has a North - South imbalance, if I understand that correctly 20% of power would be lost with a 1500km transmission distance.

Replacing all the power of Spain would be very inefficient, even if the connections did exist.

https://en.wikipedia.org/wiki/National_Grid_(Great_Britain) (Power Flow section)

[Retric]

For very long distance power transmission, losses primarily depend on voltage and AC>DC>AC conversion efficiency which have both been increasing. China just completed a 3,300 km (2,100 mi) 1,100 kV line capable of sending 12 GW at the cost of 5.9 Billion dollars.

Over 1500km you can keep losses under 10%, but building infrastructure isn’t free. East to West links tend to work better because you can time shift demand and thus build fewer power plants.