One of the biggest drawbacks to harnessing solar energy is the large up-front cost. Installing solar panels runs tens of thousands of dollars, and even with all of the savings on electric bills, it can still take 10 or more years to recoup the investment. And panels are only useful if you live in a very sunny area; if your region isn't sunny enough, you'll still have to take power off the grid. And panels require some cleaning and maintenance.
Why is geothermal energy better than other energy sources?
Answered by Discovery Channel
How does hydropower work?
Answered by Planet Green
What is nuclear fusion?
Answered by Craig C. Freudenrich and Discovery Channel