Given,
p = 1600 KW

Mass = 5kg

Initial temp T1 = -15°C

Final temp T2 = 0° C

Efficiency is 0.80

h1 is the specific enthalpy of ice at temperature (-364.18 kJ/kg)

h2 is the specific enthalpy of saturated water at temperature 2 (0 kJ/kg)

Q is the heat required to melt the ice

Let us determine the heat required to melt ice from -15 °C to liquid water at zero degrees Celsius.

We know that Q = [ h2 - h1 ]

(5 kg)[0 - (-368.18 kJ/kg)]
1821 Kilo Joules

We know that Process efficiency is equal to the heat rate divided by the heater power.

E = (Q/T)/P = Q/tP

Solving for time :

t = Q/EP

1821/(0.80)(1.600kJ)
= 1423 S
1 min = 60 s
So 27.3 mins