The calculation scheme Annex IV of the respective regulations is only provided for one solar connector connected to one tank. In cases where the solar collectors are connected to more than one tank, the sum total of the included volumes can be applied; the volume-weighted 35 standing losses and the worst energy efficiency class of the tanks should be used.
Disclaimer: Please note that the European Commission cannot provide a legally binding interpretation of the EU legislation, as this is the sole competence of the European Court of Justice. Any remarks from the European Commission services are without prejudice to the position the Commission might take should related cases arise in a procedure before the Court of Justice.