Thanks to you and Genome.
I think that the easiest solution is to use a current transducer (would need to buy one, but they seem to be not too expensive), a voltage transducer (which I already have), and a analog multiplier like AD633. It is very simple conceptually. Maybe I would amplify the current signal from a shunt (which is what I have presently), with a regular opamp, if current transducers prove more expensive than it seemed when I visited lemusa.com.
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