Wrong Track: If a car headlamp bulb has a power rating of 24 watt, this means that it always gives out 24 joule of heat and light energy each second.
Right Lines: If a car headlamp bulb has a power rating of 24 watt, this means that it is designed to shift 24 joule of energy per second at its normal operating voltage of 12 volt. However, if the voltage is reduced (say to 6 volt) then the power output will fall below 24 watt and the bulb will be dimmer than normal.
A range of power outputs
Thinking about the learning
So an electrical device such as a light bulb can have a full range of power outputs depending on the operating conditions. If the supply voltage falls, then so too will the current and the power output of the bulb will be reduced.
Household light bulbs are rated and sold in terms of their power output, but you need to bear in mind that figures of 100 watt or 60 watt apply for the normal 240 volt supply.