I am installing a radiant heat system alongside a rocket mass heater. I want to compare the cost of operating each. For the radiant heat system, an on-demand electric hot water heater is being used. I want to meter power used by this device as a way to quantify the cost of operating it.
The heater will be hard-wired and operates on split-phase 240v at 19A. What's the best way to meter its actual power usage?