You need to calculate the total loads that will be on at one time.
If your located in USA, or for reference, you can begin reviewing Article 220 of the NFPA 70: National Electrical Code®
Table 220.3(A), which lists watts per square foot for general lighting in various types of facilities, may help.
Since you already have connected load, you already have the actual total lighting load. Heating and cooling load can be added, making assumptions of what will be total load based on need of either.Next add large intermittent loads, such as elevators, trash compactors, dishwasher booster heaters.
Generally, another way of thinking, is to add all continuous loads, and then add a judgment of non-continuous loads. Your total connected load will have these continuous loads, and you need to add what the total non-continuous loads will be.
Don't forget to consider exterior lighting and signage only used at night.
Here are a few references to get you started: http://www.nfpa.org/assets/files/PDF...sues072704.pdf Demand load - Wikipedia, the free encyclopedia Commercial Load Calculations