First, you need to know what the heat load is. So to start with you will need to know the quantity (volume) of beer that you wish to cool and how much you want to cool it Starting Temp and Finishing Temp and the time period over which you would want to achieve that difference. The specific heat quantity of the beer is something you should be able to get from the web. Once the beer is at the required temperature the heat load should be minimal and will comprise the specific heat of fermentation, gains from the environment and mechanical gains from machinery i.e. circulating pumps or stirrers.