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?
12 volts
If it is AC then you don't need a formula, you just need a transformer.
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.