What kind of battery can power a 40 watt light bulb?
If the bulb is a 40 watt, 120v bulb, you would need 20 6 volt batteries wired in series, or 80 (!) 1.5 volt batteries. 1.5 volt batteries are AAA, AA, C, D, etc. Since this solution is rather extravagant, try finding a light bulb made for an RV. These typically run off of 12 volts, and you can get a single 12 volt battery, or 2 6 volt batteries, to run this bulb.
The amount of light produced is measured in lumens, while the electric power used is measured in watts. An old-type 100 watt incandescent bulb produces about 1200 lumens. The same amount of light can be produced by a 70-watt halogen bulb, a 20 watt CFL bulb or a 15 watt LED. The power that is not turned into light is turned into heat instead.