L5 Society founder, writer on space engineering, memetics and evolutionary psychology. Knowledgeable about cryonics. There is a wikipedia page about me that when it has not been vandalized might be mostly correct. Worked in electronic design, systems analysis, programming.
To put a dent in the carbon and energy problems consider a project to replace all the coal fired plants in the US, some 300 GW, in one year.
That would take 60 5 GW solar power satellites at 10,000 tons each or 600,000 tons lifted to GEO in a year or about 2000 tons a day.
Assuming mechanical power applied at high efficiency, lifting 2000 tons per day takes a little under a GW. So it takes 5 GW-days to lift the parts for a power satellite that--when it comes on line . . . . it repays the energy required to lift it into orbit in one day.
Even 1% efficient rockets would repay the lift energy in 100 days, but it's fairly clear that a high efficiency, high capacity space elevator would solve the world's carbon and energy crisis in short order.
Space is definitely the place to put solar cells no matter what cost or efficiency since they collect from 4 to 20 times as much power there.