Solar Cells contÕd
¥Payback time of energy used in fabrication
from about 1 year for roof integrated built-in installations in Phoenix made from high efficiency amorphous Silicon 
¥Over 30-year lifetime, Si based solar cells will produce 6 to 31 times the amount of energy used to produce them
¥No GHG emissions during power generation
¥Energy generation cost is ~0.18 $/kWh in Arizona (TEP)
¥Monthly use of 500 kWh per house: $90 (TEP cost ~$40)
¥TEP anticipates solar cells become cost effective ~2015
¥Solar cells on the buildings in Tucson would provide the power needed for Tucson transportation via electric cars
http://en.wikipedia.org/wiki/Solar_cell#Energy_conversion_efficiency