How many 1.5 volts battery needed to light a 15 watt blub?

Answer 1

There is no answer! It depends on the design of the bulb. You can have a 15 watt bulb that is 120 volts or one that is 6 volts. Watts is a measure of power. Volts times amps equals watts. At any given wattage, the higher the voltage is, the lower the amps and viceverca.

Answer 2

Well it really depends on the light bulb. If it is a standard 115V incandescent 15W lamp, you would have to wire 75-80 of them in series to get a somewhat normal line voltage. Incandescent lamps will run on lower voltages than what they are designed for, but will give a dimmer light. You can buy incandescent lamps designed to run off of 12V, but then you are trying to draw about 1.25 amps. I believe (off the top of my head) that I remember a AA allowing you to draw about .5 amps so you would have to wire about 24 AA batteries in a combination of series and parallel.

There are a lot of factors to the question that you are missing and it would be impossible to answer it without more details. My suggestion for whatever you are doing: go to radioshack and buy some LEDs that run on 5V or something similar.