If the car charger is purely a 240V device, or at least has practically no unbalanced (120V) circuitry, then you could build something akin to a Kill-a-watt from one of
these and a box, plug, socket, bit of wire, etc. You'd connect the voltage sense lines to the two hots, and the current sense transformer over one of the hots—exactly as though it were a 240V two-wire circuit.
If the legs are used in an unbalanced fashion, you'd need two—one for each leg, with the voltages each referenced to neutral—and then have to add the energy consumption values together.
A used utility company electric meter would also do the trick, and old non-remote-read ones are not hard to come by for reasonable prices. Again, an appropriate box and wiring, etc. is needed.
