Judgment of the Court (Fifth Chamber) of 9 September 2021.#XY v Hauptzollamt B.#Request for a preliminary ruling from the Bundesfinanzhof.#Reference for a preliminary ruling – Taxation – Taxation of energy products and electricity – Directive 2003/96/EC – Article 17(1)(a) – Tax reductions on the consumption of energy products and electricity in favour of energy-intensive businesses – Optional reduction – Arrangements governing the repayment of tax levied in breach of provisions of national law adopted on the basis of a power granted to the Member States in that directive – Payment of interest – Principle of equal treatment.#Case C-100/20 REC.