There is. And the reason can be traced to energy efficiency…
As a typical adult, you need at least 2,000 Calories a day. Making the conversion to less arcane units, that works out to about 100 watts of power, 24 hours a day. But remember that if you got your energy through photosynthesis, you would absorb only 8 watts for each square meter of skin. Most of us have about 3 square meters of epidermis, roughly half of which is in shade at any given time (more, if you insist on wearing clothes). So that’s just over a dozen watts of daytime power, nearly 10 times less than our burn rate. To provide the energy for one day’s worth of your gusto-grabbing lifestyle, you’d need to bake on the back patio for three weeks.” (Yahoo! News)
