Article on positioning Solar Panels on homes

[Devonian]

I've been thinking about an ultra portable system such as this:

http://www.amazon.co.uk/dp/B00K0DBGTG/re...SQEB&psc=1

Unlike most foldable panels, this systems outputs in 18v and 12v as well as the usual 5v USB. This means it can be used for charging deep cycle leisure batteries for running small 12v power systems as well.

Does anyone know how well these sorts of systems work if they are suspended say from a curtain track inside the window? I accept they will not be as efficient, but is it a significant loss to have it behind the window glass?