Can anyone help with physics...?

Question

in a heating system the temperature of water from hot tap is 15'C , how long will it take for the water to reach 60'C?

[water from tank flows to a thermally insulated 100 litres capacity system, heated at 240V with resistance 45ohms. density of water is 1000kg/m3, specific heat is 4.186kJ/kg.'C, gravity 9.81m/s2]

Answer 1

Energy required = mass x sp.ht cap x(T2-T1)

E =100kg x 4186 x (60-15) (1 litre has mass of 1Kg)

E =18837000J or 18837kJ.

1 watt = 1 joule per second

Current in heater = 240/45 =5.33 amps.

Wattage of heater = 240 x 5.33 = 1280W (= 1280J/s)

Time taken = 18837000/1280 = 14716 seconds.

= 245 minutes = 4.08 hours.

Seems a long time...I'd get a bigger heater.