From what formula to calculate how much light a 12V 20W bulb will glow?

Colleagues, what formula or law to calculate how much a 12V 20W bulb will shine on a 55Ah battery

First, we calculate the current drawn by the bulb:

I=P/U
I=20/12 ?1.7A

Now we divide the battery capacity by the current:

55/1.7 ? 32.36 h

So the bulb will be on for about 32 hours and 19 minutes.

from ohm's law

I=20W/12V, i.e. about 1.7A

battery gives 55h after 1A so 55h / 1.7A gives about 32.5h

But that's just theory, in practice it's different.

Assuming, of course, that the battery will maintain a voltage of 12V all the time. In addition, after some time, its voltage will drop enough that the bulb will stop glowing, but the current will continue to flow. I don't know if it's possible to calculate exactly how much it will shine. And by the way, capacitance is defined unless the voltage drops below a certain limit, what is this limit?

The ambient temperature is also important for calculating the battery capacity. best for acid batteries 20-25 degrees C.
Regards

It is not so easy to calculate. The bulb at 12V power supply has about 7.2 ?, but at 11V it will have a different resistance, e.g. . In this way, it is impossible to judge how much such a bulb will glow, precisely enough to use the formula and stick to it strictly. What kind of light bulb is it? Because if it's different, these dependencies will be different. An ordinary filament bulb has a resistance that depends on the temperature of the filament, i.e. on the supply voltage. It is not a linear element, it is rather a thermistor in terms of resistance. And the moment when you can say that it is lit and that it is not lit is not a matter of the moment (in this case the power supply) but a contractual matter, so for one it is a weak glow and for the other it is just glowing.