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How much energy is required to boil off 3 litres of water from 20*C?

This is not really homework, but I still thought this would be the best place to post:

To get something to compare to in terms of energy (joule/watt), I would like to know approximately how much energy it takes to boil all the water in a standard kettle (3 litres).

I tried doing this with data from log p H (mollier) diagrams for water:

Enthalpy of water at 20 *C and athmospheric pressure from mollier: 86,6 kJ/kg

Enthalpy of saturated steam (100*C): 2676,1 kJ/kg

DeltaH= 2676,1 - 86,6 = 2589,5 kJ/kg

3 litres of water = 3kg so: 2589,5 kJ/kg * 3 kg = 7768,5 kJ required to boil 3 litres of water.

However when searching for this on google i only found another solution to the problem that did not match my answer using constant specific heat capacity and heat of vaporization:

http://wiki.answers.com/Q/How_much_energy_is_needed_to_boil_water#ixzz18El72mGe

However according to wikipedia: "The specific heat capacities of substances comprising molecules (as distinct from monatomic gases) are not fixed constants and vary somewhat depending on temperature."

So using a constant heat capacity value from 20 *C to 100 *C would be wrong, no?

Are any of these methods viable? Which is better?They don't match up when i use his equation for my case (although I know it's partly because we have different values for heat of vaporization and it seems like he has mixed up some units in his description).

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# Energy required to boil water

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