17:24 Mar 19, 2011 

... it's really difficult to decide whether "capacity" equals "power output" (as implied in Bernd's answer) or "capability" (as implied in my answer). The exact meaning of "turn down" then depends on the interpretation of the former.

Personally, from I gather, they are referring to the ability/capability/flexibility to turn down (to reduce output) according to demand changes. The term is used in this way here, in relation to a combined cycle gas turbine installation (but using "capability" instead of "capacity"):

http://www.freepatentsonline.com/y2010/0175385.html

It will be interesting to see which is the correct interpretation. In retrospect, more context would have been necessary -- but ok.