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I used a single power supply for an op-amp, and I could see the current draw on the power supply. It was easy to determine power consumption using P = V * I.
How about dual power supply (+-9V); to calculate the power consumption do I add the supply: 18V instead of 9V when calculating power consumption.
Yes. The appointment of a "0V" reference is an arbitrary one for the benefit of humans. As far as the op-amp is concerned, it has 18V across it and some I flowing through it, and P is always V * I.
Where you need to be careful though, is in considering the current consumed by loads connected to the op-amp, rather than the op-amp itself. If those loads are not referenced to the negative rail (eg. they're referenced to ground instead) then they may not have the full 18V across them and will need to be treated as such.
