It takes approximately 2 . 109 J of energy to manufacture 1 square meter of crystalline- silicon photovoltaic cell. (Actually the number quoted was 600 kWhr. Can you figure out how to convert kilo-watt hours into Joules?) Assume that the solar cell is 12% efficient, and calculate how long it would take, given 250 W/m2 of sunlight, for the solar cell to repay the energy it cost for its manufacture.

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