Open display counter vs. Closed display counter

Hey,

First a little introduction - I'm a former electrician and now I'm studying Energy Technologist. As a part of my education, I have to make energy analyses of different buildings. Now I'm working on a supermarket; I have made the calculations on walls, lighting, user behavior etc. BUT I'm stock with the calculations on the display counters. It is some old display counter with open tops. The customer is only interested in solutions with a short payback, so the only solution will be to make the display counters to closed tops.
And now my problem occur! I don't know how to calculate the energy saved by closing the display counters. I have tried to use Cooltools-A - Refrigerated display cabinets, but when I plug in my info, it looks like, that there is no savings by choosing closed display counters.
Is there another program - or an equation - to solve my problem?

Thanks in advance.

ur air infiltration is the same for both! surely just deduct the 2kw load

Ahh okay, I can see your point! But the only way I can change it, is by changing "QE,Cat" - and that is still the same closed or open, the only difference is the lid . Or?

Thanks for reply.