The current is based on the resistance of the light bulb. Power is the product of voltage times current.

Say for example, you have a bulb running at 3.5V and it consumes 1A. Its internal resistance would be equal to 3.5V/1A= 3.5 ohms. The power consumptions would be 3.5V times 1A, for 3.5Watts.

Now take 1.5V and based on a resistance of 3.5ohms, you would have 1.5V/3.5ohm for a total current of .43A. At 1.5V and .43A, total power would be .645W