AC power supply issue
It's not clear what you are really asking, but in general, think of the power supply as being a Thevnin source.  If you don't know what that is, look it up.  It's a standard term that is surely well described out there.

Basically, to first approximation, you have:



When the power supply is open circuit, no current flows thru R1, so there is no voltage across R1, so the power supply output is just V1.  When the load draws current, R1 will drop some voltage, and the power supply output is V1 minus the voltage dropped by R1.  That voltage drop will be the current times the resistance of R1.

By knowing the current the load is drawing at whatever voltage it is actually given, and knowing the open-circuit voltage, you can compute the value of R1 by using Ohm's law.  You can derive the load current from the voltage presented to the load and the power it is drawing at that voltage.