Source: Science20.com
News
Aug. 2, 2012
If you use solar power to charge your iPod, it requires an array of panels, it takes a long time, and you will need to do it again in a day.  That's not a knock on solar energy, the efficiency of solar power panels are pretty good, about 8 percent; by comparison even plants using photosynthesis are only 5 percent efficient and it's hard to argue with plants and photosynthesis.  But efficiency alone is not painting an accurate picture.  The energy density of gasoline is much, much greater.  For perspective, Ken Cohen of ExxonMobil (I kid you not, they have a blog - it's pretty interesting too) notes that a gallon of gas has enough energy to charge an iPhone every day for almost 20 years.
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