How do I determine the cooling capacity needed for a specific application?

I know the item I want to cool, it's two large bottles of water. Refrigerant is R-143a. The temperature I want in the fridge is 38F. So, based on the 3 variables of ambient temperature, thermal load and fridge temperature how do I determine the cooling capacity I need in Watts?

Is the thermal load simply Q=m*cp*deltaT of the bottles of water?

I know that to size a compressor, the cooling capacity is given for a specific temperature at the evaporator. I know that it needs to be less than the temperature I want in the cooling cabinet (38F), but how much less?