Theory is P = V x I and V = I x R. Practise is the same.
I have connected a 21W bulb across my car battery. Engine is off, Voltage 12.15v and current 1.78A.
Turn on the engine and voltage is now 13.66v. Current increases, because resistance of bulb is still the same and V = I x R, to 1.90A
Wattage of a bulb is stated for a specific voltage and for car bulbs that is 12v. Increase the voltage, you also increase the current and the wattages goes up. In this example the 21W bulb is now drawing 26W from the battery when the engine is running