How do I calculate Watts for a dual rail power supply?
How can I calculate power consumption?


Calculate power consumption for each individual circuit (as if you had wattmeters in each seperate power feed) and add the numbers up. For instance: -







simulate this circuit &ndash; Schematic created using CircuitLab

You could calculate power as 20V across a 20 ohm resistor and get 20 watts BUT you don't necessarily know that currents from both supplies are the same. So, you should look at it from the perspective of two individual power supplies feeding their individual currents into a black box (and that black box might just be disposing of some current to ground). 

Either way you'd still get the same result, 20 watts, for the example above. 

You could just imagine it as two 10 ohm resistors each connected to 0V and, if you do the math and you would get the same result but, what if those two resistors are unequal - you are forced into summing the individual powers based on individual voltages and their individual respective currents.