My understanding of the Prius AC compressor is that it has an electric motor in it (hence a hybrid in itself) that can run the compressor when the ICE is shutdown (clutch disengaged of course). The reason they still drive the compressor with the ICE at all is probably that it's more efficient to use surplus ICE horsepower than exclusively electric.
Are you sure that the Prius AC is driven by the ICE also? That was not my understanding. I would go downstairs and look but there is so much jammed in that engine bay that you can't see much.
FWIW, the Prius compressor motor draws 1600W at it's max setting. I dont haved my slide rule on hand to convert to HP.
fyi eCycle seems to have a buck boost regulator for Prius use to add energy to the stock battery from an additional lower voltage battery pack in increase the electric battery range. Will try to get there within the next week for their inventory clearance sale.