What amp rating ESC do I need?
This is why we have Ohm's Law:  I = P/E


P = Power (Watts)
E = Electromotive Force (Volts)
I = Current (Amps)


You can calculate any of the factors given two factors:



I made an Ohm's Law calculator online here: http://www.rcrowley.com/eirp.htm

If you plug in 30V and 1200W, it shows that the current is 40A.
So your 50A ESC should be adequate.