Brad Plumer looks into (for Vox.com) the current best estimations as to when, exactly, solar power will become economically viable enough to supply a substantial proportion of our power needs:
[...] the industry should set a goal of pushing the installed price of solar to $0.25 per watt by 2050 — down from around $3 per watt today. That's a mind-bogglingly low number, and it could require thinking about solar innovation in a radically new way. Current approaches to cutting costs won't necessarily get us there. We may need experimental new technologies. Or novel ways of integrating solar into our walls and windows. Or robot installers.
The article is a great read, and demonstrates that beyond all doubt, we still have a very long way to go before solar power really becomes a feasible option. It's sad to think about just how long this has been the case–we've all been saying the same things since the 1970's.
Despite these sad thoughts, it's important to remember that adoption of solar generation is growing far more rapidly than ever before. Sooner or later, there will come a tipping point, and solar will start to seem ever so slightly more attractive, from an economic point of view, than one or more other methods of power production. Suddenly everyone will find it sensible to switch, and a snowball effect will take over.