A water cooler of storage capacity 120 liters can cool water at a constant rate of P watts. In a closed circulation system (as shown schematically in the figure), the water from the cooler is used to cool an external device that generates constantly 3 kW of heat (thermal load). The temperature of water fed into the device cannot exceed 30° C and the entire stored 120 litres of water is initially cooled to 10 °C. The entire system is thermally insulated. The minimum value of P (in watts) for which the device can be operated for 3 hours is (Specific heat of water is $4.2 \mathrm{~kJ} \mathrm{~kg}^{-1}$ and the density of water is $1000 \mathrm{~kg} \mathrm{~m}^{-3}$ )