If you want to convert 24v AC into 12v Ac, it's simple. Just get a transformer to convert 24 volts into 12 volts. This is called a stepdown transformer.
If you want to convert 24v DC into 12v DC, it's more complicated. First you have to change the 24v DC into 24v AC by a switching converter that changes DC into AC. Then you would use a stepdown transformer as in the former example. Once the 24v AC is converted to 12v AC, then you use a rectifier bridge to change the 12v AC into 12v DC.
No. Design and testing ratings are there for your protection. I would not use 24 volts on a rated 12.
20.4 volts.
To convert watts to amps, you can use the formula: Amps = Watts / Volts. In this case, to convert 200 watts at 12 volts to amps, it would be: 200 watts / 12 volts = 16.67 amps. So, 200 watts at 12 volts is approximately 16.67 amps.
Volts cannot be converted to amps. If you mean convert 220v to 12 volts output, then you need a 220v to 12 v step down transformer with the VA or watt rating you require.
Connect them in Series and you will get 24 volts but the amperage will be the same as 1 battery.
If it is AC voltages you need to convert, you need to find a transformer to do this job. Without knowing the application it is hard to guess what you are trying to do.
No, a 220 volts AC fan cannot run directly from a 12 volts battery. The fan requires a much higher voltage to operate efficiently. You would need a power inverter to convert the 12 volts from the battery to 220 volts AC to power the fan.
5000 volts
To what?
If it is AC then you don't need a formula, you just need a transformer.
12 volts
12 va These are two different entities. Its like asking how much water will flow with pressure. With the following formula W = Amps x Volts you can see the relationship they have with each other, the end product resulting in Watts.