What causes lamps to have different wattages?
The operation of an incandescent lamp is simple and self regulating. When electricity passes through a lamp filament it heats until it glows (incandesces) and produces light. The amount of light depends on the amount of electric current that passes through the filament. The length and size of an incandescent lamp’s filament limits the amount of electrical current passing through the lamp, regulating its light output.