Case C-305/17: Judgment of the Court (Fourth Chamber) of 6 December 2018 (request for a preliminary ruling from the Okresný súd Bratislava — Slovakia) — FENS spol. s r.o. v Slovenská republika — Úrad pre reguláciu sieťových odvetví (Reference for a preliminary ruling — Free movement of goods — Customs duties — Charges having equivalent effect — Levy on the transmission of electricity generated domestically and intended for export — Compatibility of such legislation with the principle of free movement of goods)