Extracted from another thread tha had got a bit messy. The immersion heater in an electric hot water system is rated at 9 kW. If the tank is initially filled with 40 gallons (about 150 litres, with a mass of 150 kg) of cold water at 15 oC, how long will it take the heater to raise the temperature to 40 oC? Ok this is as far as I have got: Q=CmdeltaT deltaT=40-15 = 25 q=Q=4196*150*25 15735000J Ok so now I need to get time. p=deltaq/deltat So to get time do I just say time=p*delta q ???