I did a bit of research to find out how efficient the Toyota Prius electrical system is.
For now I have had to rely on generic information, not Toyota's
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The coulometric charging efficiency of nickel metal hydride batteries is typically 66%, meaning that you must put 150 amp hours into the battery for every 100 amp hours you get out. The faster you charge the worse this gets.

This is interesting but I wonder if this is for full charge. Typically full charge has higher losses and partial charge can be very efficient such as in lead acid. Since the Prius uses "micro cycles" I would imagine the battery to be more efficient than 66%
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Using a C/10 rate for charge and discharge I have found that almost every NiMH battery yields a ~98% cycle efficiency.

At which rates does the Prius battery typically charge and discharge? I guess this could be figured from the Graham scanner by graphing battery current in and out.
My guesstimated electrical efficiency of the THS from ICE (35%eff) to MG(90%Eff) to battery(75%Eff) to DCDC(85%Eff) to 12V?
So a 450 watt load requires very roughly 711 watts to produce from the 35% efficient engineMGbatteryDCDC12v. Some of that is recovered energy though so there are too many variables to be accurate. For instance, It would be much more efficient if it came directly from the MGs.
How does it compare to a regular ICE(20%Eff) belt(?%Eff) Alternator(45%Eff) 12V? It's better. A 450 watt load requires roughly 837 watts from the 20% efficient engine.
Still, in both cases, the running electrical load is a substantial drain on the system.