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.
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.
1.5kva has enough electrical power to supply 100 volts of electricity that is at 15 amps. You can also supply 300 volts at 5 amps and 1000 volts at 1.5 amps.
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.
Connect them in Series and you will get 24 volts but the amperage will be the same as 1 battery.
5000 volts
To what?
If it is AC then you don't need a formula, you just need a transformer.
Change the trailer light bulbs to 24 volts and supply a 24 volt source to power them, in that order.
12 volts
Yep. It is called an inverter to convert 12 volts to 110 volts AC. Get one that will supply the amp needed for the microwave.