A water heater of power 2000 W is used to heat water. The specific heat capacity of water is $4200 \mathrm{~J} \mathrm{~kg}^{-} { }^1 \mathrm{~K}^{-1}$. The efficiency of heater is $70 \%$. Time required to heat 2 kg of water from $10^{\circ} \mathrm{C}$ to $60^{\circ} \mathrm{C}$ is $\_\_\_\_$ s . (Assume that the specific heat capacity of water remains constant over the temperature range of the water).