>>If the Chinese line cost 7 billion dollars, we could scale this up by size and call it 4.5 trillion dollars.

Undersea power lines are significantly more expensive than land ones - expensive enough that for many offshore wind farms, the major cost is the power lines. Deepwater submarine power cables cost more. That's political considerations to the side (consider Germany's post-1990 dependence on Russian Gas).

Also, there's the ocean 'electrome' to consider. Many marine animals are sensitive to electrical fields - sharks attacking optical cables (which are bundled with electrical cables to power repeaters) is a problem.

Bright side: stationary batteries don't have the same constraints as the ones for vehicles or phones. Iron redox (both air and flow) batteries look promising.

Just my $0.02