1 watt is 1 joule per second. The heating element is rated at 500 watts, but the battery can only deliver 12×40 = 480 watts, so I assume that that is all that is available for heating. If the figure of 41.86 kJ is correct then the time taken for the heating element to deliver this amount of energy will be seconds. (That's a bit less than one and a half minutes, which sounds a bit fast to heat up that amount of water. But we are assuming that no energy is lost, and that the entire 480 watts goes into heating the water.)