We know that the formula of heat is given by:
Heat, H = I2 Rt
Where,
t = 30 min = 30×60 sec = 1800sec
I = 15A
R = 500Ω
∴ H = (15)2 x 500 x 1800 J
H = 225 x 500 x 1800 J
∴ H = 20.25 x 107J
Thus Total heat produced by the heater is 20.25× 107J in 30 min due to flow of 15 A current.