The energy consumption of a fridge depends on what it takes to cool the entire contents within a fridge, because heat from center of the food will travel to the surface when the surface cools down. Till the temperature of the entire food item matches the set temperature of the fridge, the fridge will be running. When you open the fridge, warm air enters the fridge, and the contents of the fridge gain heat.

Then there's variability depending on thermal conductivity and specific heat capacity of various foods. So what you're trying to calculate is heavily dependent on the actual contents, not just surface area.

Of course, fridges have cycling behavior similar to an AC that turns on as soon as temp dips below a set temp, but has a window of sorts. Making the above math even more complicated.