Energy Matters Forums

[franks]

Hi,
My inverter is rated 1800w it spits the dummy trying to power with a typical 2400w electrical kettle
I beleive there used to be a small kettle 700ml capacity using 1500w I understand with a typical boil time of 90secs it would comsume 35w/h , (my system can support 2400w/h over the weekend) handy for a quick cuppa rather than fire up the primus gas stove.
Anyone know if small electrical kettles are still around, I recall somewhere used to be sold kambrook brand?
Regards
Frank&Bev

[MadMax]

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=61680

Heating 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

[Sandivee]

Drink cold water?

[andarm]

There is bo need to add the latent heat of vapourisation value as it's 100 degree water you're after rather than 100 degree steam.

[MadMax]

very good point andarm! i got a little carried away ... it's still quite a bit of energy.

[nicko]

What about using a small microwave to boil a cup of water in around 2 minutes compared to boiling several cups in a kettle. Surely that would be more efficient?

[MadMax]

total amount of energy required to heat the water would be the same ..except you're adding the possible inefficiencies of an inverter (you'll need quite a powerful one) and the microwave (no idea how efficient these are)

max

[andarm]

You'll lose at least a feww hundred watts in a microwave. i.e. an 1100w may draw 1300ish watts.

[Joey]

If you wanted to be a bit naughty you could wire 2 standard kettles in series to effectively reduce the current draw to half , but the boiling time would be about 2.5 times longer.

While on the subject of kettles a lot of people overlook the amazing Eco Kettle , the kettle that has a storage chamber and a separate boiling chamber which allow you to only boil the water you need.
IE: if you want to boil one cup you push the button on the top and it transfers one cup to the boil chamber which is marked with different levels , it only takes about 20 seconds to boil and no wastage heating water you don't need.
It does draw 2200W but not for long I think Energy Matters would sell truck loads of these if they stocked them.
Everyone that has seen ours wants one , so I have bought 17 last year .

[Bthree]

You'll lose at least a feww hundred watts in a microwave. i.e. an 1100w may draw 1300ish watts.

Boiling water in a microwave has its merits, just boiled a 250Ml cup of water from 15deg C which took exactly 2 minutes in an 1100w microwave.

So 1300w divided by 60 = 22watts .... multiplied by 2 = 44 watts

Quite efficient really and does not exceed the 1600 watt limit

A microwave cooker is recommended to run on a sinewave inverter not modified signwave.