Re: Power Query

[Marc_F_Hult <MFHult@xxxxxxxxxxxxxxxxxxxxx>]

On Thu, 01 Feb 2007 22:54:35 -0500, Beamer Smith <beamer@xxxxxxxxxxxxxx>
wrote in message  <jcc5s2lj3fi2hu3jek410b0roi234hk8rp@xxxxxxx>:

>A bit of a stretch for this group but the experts seem to be here.
>Our building wants to install seperate meters for our power in part to
>mitigate the expence of the condo's  power bill.
>The building is 208V  but most abbliances are 220. I know for example
>that my mother can boil a pot of water in about 5 minutes on her 220
>service and 22 stove (not new or efficient)  but on my 208 service and
>220 stove it takes much longer.
>Q: Am I paying more to boil a pot of water or not?
>Some say that the meter doesn't work that way and that if I had a 12V
>service and 24 hours to boil the water it would be the same as 208 for
>much less time..
>Help me out here folks...
>

Bill and Lee are correct in the assertion that you pay for watts not volts.

But that's not the whole answer. The Tea-kettle question is an classic
example of an energy budget which is parallel/similar to a mass balance
problem.).

Think of it first with the close analogy of a kettle with a leak being
filled with water (mass balance):

RateWaterIn - RateWaterLeaked = Change in amount of water stored in kettle

Note whether you ever fill the kettle depends on BOTH rates. Too large a
leak (hole) and the kettle never fills.

For making a  kettle boil,  the budget is:
For temp < 212F

	Heat In - Heat Out = Change in Heat stored in kettle

Heat Out ("lost") is a function of:

	Time,
	K, Thermal conductivity of the kettle material
	Temperature difference between kettle  and environment


So if you reduce Time, you reduce heat loss.

So the 12 volt element may never get the water in an uninsulated kettle to
boil, the 208 is less efficient than 220 volts and the most efficient is
INSTANTLY applying enough heat to boil the water because there is not time
to lose any heat.

If you reduce thermal conductivity of the kettle walls (i.e, insulate  the
kettle as is done with some electric kettles) you reduce heat loss.

Alternatively, you could raise the room temperature to 211 F to minimize
the delta T heat loss. Hard on the heating bill among other things...

... Marc
Marc_F_Hult
www.ECOntrol.org