How much difference is there in energy use between the different energy efficiency classes?

This varies slightly between different product groups. For refrigeration appliances, a product with C may use more than 50% more electricity than a product with an A.

More info here.

^{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}