Typically, what percentage of power consumption of a z14/z15/z16 machine that's running flat out is used by the same machine which is switched on, but not actively processing any useful work (i.e. all the subsystems and started tasks such as CICS, DB2, MQ, IWS etc. are up and running, ready to receive work, but there isn't any work to be received)? Perhaps the situation you might get in the middle of the night on a development machine when the developers aren't working 24x7?

Can we do any better than the non-answer of "it depends"?

The best way would be to undertake a direct measurement or have some way of measuring the power consumption over time of your specific machine.  Comparing watts used at idle and at peak operation would let you make some assessment.  Alternatively i'm not sure of this is a feature of the machine that you can track power separately with some monitoring?  I'm not overly familiar with the Z series but they are promoted as sustainable and efficient and tracking consumption would make a lot of sense.

The following PCF guide for the z16 has a lot of detail that may assist you, there are PCF guids for the other models as well.  Being an IBM document it does say the PCF is to be used as a guide only.  Interestingly 75% of the PCF comes of from the use of the machine, implying 25% is production and EOL.
https://www.ibm.com/downloads/documents/us-en/10c31775c6d40084

You could also log a support request with IBM.  I'm sure they would at the very least be able to provide the base vs peak consumption.  It's not a direct answer but hopefully it helps you get the information you need.