I think you don’t understand the situation. In these cases the power comes from an external brick that supplies +16 volts. The internal “power supply” is not a power supply. Instead the internal system generates +12v, -12v, and +5v from the 16v input. There is definitely a power cost for this conversion!
Knowing the current draw would be helpful. For example, if you had to replace the brick, if you used current of just the modules in the case you’d be short on power.
In my case, I’m running this case from a battery. I can calculate the battery life from the power used by the modules, but that number isn’t accurate. Battery life will definitely be shorter. If I knew the power consumption of the case I could calculate battery life more closely.