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

If I have a 5V power supply powering a circuit which is connected to 0V, to calculate power usage I would take the total current in Amps and multiply by 5V.

However, my current circuit uses -12V, +12V (for an op-amp) and 5V, 0V for the rest of the circuit. How can I calculate power consumption?

• Sigma(Current via each battery * voltage across the respective battery) must do.. Apr 28, 2016 at 17:27

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 – 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.

• How do I calculate the power consumption for a +12V and -12V power supply? Would I use 24V in the V*I equation? Apr 28, 2016 at 17:28
• You cannot guarantee thet the current taken from one half of the supply isn't returning via 0V and therefore not seen by the other half of the supply so play safe and do it individually for each rail. Apr 28, 2016 at 17:29