The level of insulation offered by any given window is made up from a combination of the insulation of the frame, and that of the glass or ‘sealed unit’ that is used. Other factors such as frame size and air leakage are taken into consideration, so all in all it’s quite a difficult calculation to understand.

That’s why the government has introduced the Window Energy Ratings scheme. By using the same ratings system that items such as fridges and washing machines have been using for years, it is now possible to rate a window from any given supplier. The finished window combination is independently tested, and awarded a rating from A to G.

Those achieving a C-rating or above demonstrate they have been designed, manufactured and tested to provide a good level of overall thermal efficiency. So, if C-rated is energy efficient, how much better does an A-rated window perform?

Conout9:

Looks like you found a reference to some British ratings.

For more info try:  http://www.bfrc.org/

In general, the British have adopted a scheme similar to the Canadian Energy Rating.  It accounts for both the losses and the gains over a heating season.

As you said their ratings go from G to A.  Here is a list of what that means at the upper end of the scale:

a C Window is between -20 and -10  W/m^2
a B Window is between -10 and 0  W/m^2
a A Window is a net gain, that is 0 or greater

see their website for the complete list

The formula that is used to calculate these numbers is also on the BFRC website.  If you can tell us a bit about your windows, then i can give you an educated guess as to how they might compare.....

Unlike many products, the energy-efficiency of fenestration products varies according to climate. Features that make windows energy efficient in one climate type may offer little benefit in another. The energy efficiency of fenestration products is primarily a function of the U-factor and Solar Heat Gain Coefficient (SHGC).