It might be hard to find how labels E, D, C, B, A, A+, A++ are actually calculated. Herefore we created a small tool using the documents available from the European Union; regulation No 874/2012 in particular. (Scroll down to get to the actual calculations.)
The columns have the following quantities:
Do not get too wind up about exact labels. Although the difference in A+ and A++ is still significant in terms of energy efficiency, it does not touch upon lifetime or other aspects of the product.
Go to the Google spreadsheet to play with values yourself. You will see that labels stated by webshops are often not right!