Hello, it seems your goods are exempted under "non-conventional energy devices or systems specified in List 8" as per sl.no.332 of Notification no. 12/2012 CE dt.17.3.2012. In as much as the exemption is unconditional, you cannot opt to pay duty on the systems manufactured by you and thereby take input credit, in terms of Section 5A of the central Excise Act,1944. In terms of Rule 6 of Cenvat Credit Rule, 2004 credit can be taken only when the final product is dutiable. Hence, there is no way u can claim the input tax incidence borne by you. However, if u are into exports, u can get the rebate of such tax amount by following the procedure prescribed under Nofn.43/2001-CE(NT) r/w Rule 18 of Central Excise Rule, 2002, or r rule 5 of Cenvat Credit Rule, 2004. There is no case law in the matter since it is the basic concept of input credit. For any further clarification, plz mail to [email protected]