This would be a great question to work on; but the answer completely depends on what temperature the water starts from, before you start shooting the heat to it.
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If the temperature of your water is zero Celsius, then it takes 4,186 joules of energy to heat 1 liter of it to 1° C. "Watt" is a rate of delivering energy, not an amount of energy. 1 watt means 1...