@Rock2 All good points. Earth reflects about 30% of solar radiation back to space, so what’s available for collection on dry land amounts to maybe 20% of the original figure – a mere factor of 5 reduction. According to the Wiki article I cited earlier, the world’s biomass consumes a mere fraction – less than 1% – of what it receives in solar energy. World energy consumption by humans (measured in exajoules per year) amounts to an even smaller fraction.
Storage & distribution is a formidable problem. None of this makes sense without technology capable of supplying power at night from robust batteries or other energy storage devices. Room-temperature superconductors (if such is possible) might revolutionize power transmission, but present-day methods might suffice if the energy source is plentiful enough.
While it’s presently inconceivable to harvest solar power at sea, future technology has no such limits. How about some kind of vast oceanic artificial photosynthesis and skimming operations? How about huge thin-film collectors orbiting in space? I lack the imagination to foresee what’s plausible, but I know that the planetary energy budget is overall favorable to supplying mankind’s needs for some time to come, if we’re clever enough.
For a really far-out energy solution, see Dyson sphere…