Something like this is extremely hard to calculate. You could sit down with Manual J and calculate out all the BTU losses to come up with a fair estimate of how many BTUs you have to replace to maintain temperature, and thus how much enenergy you have to throw into your chillers, but you probably don't have any of the information availible to do those calculations. Also adding to the cost would be heating bills for the "normal" area of the supermarket, which will have increased energy costs to overcome all the heat losses into the freezer.
As an electrician, your best bet is to just put an logging ammeter on it for a while.
