Most of today's renewable technologies have decent energy buy-back times - this is in part due to improvements in materials, and in part due to manufacturing infrastructure energy investment being paid off - and of course the increasing use of renewable energy for the manufacture of renewable energy production goods.

That all said, a lot of what is currently installed will probably never see energy buy-back. Rather, we have to look upon a lot of existing renewable investment as just that - investment - for this has paved the way for actually energy positive renewable energy technology, which we now have, and seem poised to squander.