One can estimate the energy payback time of a solar panel (that is, the length of time for the panel to yield more energy than was consumed in its manufacture) fairly easily.  But the energy invested to make the panel is a “down payment” – all at teh beginning of its lifetime – whereas the energy it yields back is a trickle that flows over many years.  Because the global solar industry is growing at a rapid (exponential) rate, it is not obvious that the industry as a whole has yet been a net energy benefit.   This paper gets to the bottom of that question.   Its conclusions might surprise you!

Dale, Michael, and Sally M. Benson. 2013. “Energy Balance of the Global Photovoltaic (PV) Industry – Is the PV Industry a Net Electricity Producer?” Environmental Science & Technology 47 (7): 3482–89. doi:10.1021/es3038824.

You can access the paper online using your PSU credentials