How can I meter power usage by an on-demand electric hot water heater?
I went with a 120/240 Volt Pass-Through kWh Meter, 3-wire (2 hots, 1 neutral), 100A, 60Hz, EKM-25IDS.

The meter is hard-wired in front of the hot water heater and provides a digital kWh readout for the hot water heater's power usage.