Question-and-Answer Resource for the Building Energy Modeling Community
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If you are modeling the whole building, then you should definitely add the apartment units to common areas. However, you can take different approaches because, to my understanding, you don't have the utility bills for all the apartment units.
If all of them are using the same HVAC system, have similar space use (e.g., residential), etc. then I would suggest calculating the EUI (energy use intensity) for those units that you have their utility bills and apply that to other units and sum them up. This would be helpful if the units vary in square footage. Also, if you have data on the occupancy of these units (how many people live in each unit), you can scale the EUI up/down for units with similar square footage but different occupancy. There are other approximations you can perform as well but it depends on the model, the building, etc.
