My Daikin's 1.5 ton AC is 2200watts. The calculation is simple,
2200w= 2.2KW
If you run your AC for one hour, your electricity meter will count it as (2.2KW x 1) =2.2units.
In your case you run it for 12 hours, 2.2(KW)x12(hours)x30(days)=792units
The rest calculation you can do by multiplying the units will slab rates on your bill. Simples