The average incoming solar radiation, counting the hours of night, is 342 W per square meter, which gives 342 MW per square km.
So you'd need about three square kilometers of collector to gather a gigawatt-day of solar energy per day. That's if you don't have clouds in the sky.
If your process is 30% efficient *, you'll need ten square kilometers for a one gigawatt power plant, and a storage device to supply the hours of darkness with the energy you collected during the day. Since it only runs half the day, you'll need twenty square kilometers to collect one gigawatt-year per year.
* Photosynthesis is not nearly as efficient as that.
According to Wikipedia, the whole Earth, (which has a cross section of 127,400,000 km2), receives power of 1.740 x 1017 MW from the Sun,
and the amount of energy captured by photosynthesis is approximately 100 terawatts: which is about six times larger than the power consumption of human civilization.
A terawatt is 1000 gigawatts, i.e. a million megawatts. That's 1012 MW. A hundred terawatts is 1014 MW. It follows that photosynthesis over the entire Earth converts less than one part in 1017-14 of the solar energy the entire Earth receives. That's one thousandth.
Admittedly, photosynthetic organisms do not blanket the entire sea and earth. But they would if they could.
There are places where solar power is dependable, and available when there is a load to be met. The load is air conditioning, and one of the places is Southern California.
There is some good information at Southern California Edison's (SCE) website, especially the
500 megawatt parabolic solar collector farm in SCE's domain.
I hope that it succeeds, but the technology is radical, .
But even there, it takes 8 square miles for a gigawatt, and of course for half of the 24 hours the output's zero.
Aha! There's another, BrightSource, planned at 1300 MW.
First stage, 100 MW, expected to be online in 2013 to produce 286,000 megawatt-hours in a year.
That's 100 MW of plant producing just over 32 MW-years in a year. The full 1300 MW should produce 3.7 billion kilowatt-hours in a year. i.e. 3.7 million megawatt-hours, which is 422 MW-years in a year.
It's great, but not enough to replace a 1000 MW coal burning plant. And we desperately need to replace the coal burning plants that supply half of our electric energy consumption.