Hey Frank,
Have a read of this post you, may want to consider sticking to gas, a fire, or a thermos of ready made tea:
http://ausubaru.com/forum/showthread.php?p=61680Heating water is a pretty inefficient thing to do using electricity, esp. when using a battery system. It depends on the start and end water temperature and the amount of water to be heated. The more water you want to heat the more energy it takes, and the colder the water the more energy it takes to heat it.
The exact amount of energy is given by the specific heat of water (4.186 Joule/gram °C).
To heat up one cup of water (250ml=~250g) from 25C to 100C requires:
250 x (100-25) x 4.186 = 78487 Joules
But water at 100C is not necessarily boiling, to boil it requires more energy. This is given by the heat of vaporization, which for water is 2261 Joules per gram.
250 x 2261 = 565250 J
Add the 2 together and you get the amount of energy required to boil one cup of water from 25C.
565250 J + 78487 J = 643737 J
Now ... lets make it relevant for people with batteries. 1J = 1Ws = 1/3600 Wh.
So, boiling 1 cup of water requires: 643737 /3600 = 179 Wh and partially boiling water requires somewhere between 21-179 Wh. (assuming no heat is lost the surroundings - that's why water always boils faster with a lid on).
If you have a 12v battery boiling 1 cup of water will require between 21/12 = 1.75 Ah and 179/12 = 14.9 Ah. This is assuming your battery is 100% efficient (and it isn't).
Moral of the story ... using your batteries to heat water is far from ideal.
max