AC, I now realize I missed one of your questions --
Our pressure cooker is pretty standard, although it is two-walled with a vacuum. I was just planning to pour hot water into it from the thermos.
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It seems fair to view the electric plate heat-up as a fixed cost. If you are cooking a lot of stuff and/or for a long time, the fixed cost is not too bad. Heating up a cup of water would not give pretty efficiency results
I imagine for similar reasons, a high wattage resistive plate has lower overall efficiency.