How do i measure watts on single phase power?
What is the correct way to do this?


The correct way to measure AC power is using a wattmeter. This is because you cannot know (or reasonably estimate) the phase angle between voltage and current and measuring amps and volts and simply multiplying them together gives a falsely high reading in all but simple cases of a purely resistive load.

So, if you had two wattmeters you can measure the wattage entering one circuit to neutral and also measure the wattage entering the 2nd circuit with respect to neutral.

If you loads are resistive the powers will be: -


8.2 x 123.6 = 1013 watts
7.3 x 123.9 = 892 watts


Total power (if resistive load) = 1905 watts.

However, if the load is resistive AND non-linear there will be harmonics in the current waveform that don't contribute to power loss but will register on an RMS measuring current meter so here's another error source.

Best way - use a proper wattmeter.