[Greenbuilding] cost to run appliance--math help

[Corwyn]

On 11/21/2011 10:38 AM, Benjamin Pratt wrote:
> My house came with a small "Instant hot water dispenser" to keep 1/2
> gallon of water at 190 degrees f. I've had it unplugged since day one
> because i thought it was wasteful expensive to run. My wife wants to
> use it this winter and asked me how much it would cost us. Good
> question. Can anyone give me an estimate on how much it costs to
> run--versus a kettle or a microwave? The math is difficult for me, and
> i have no idea how often it would run. I suppose i could put it on a
> timer to turn it off at night...
>
> It is 115v 6.5A 750w, and has 1/2" or so expanded foam insulation but
> not complete on all sides.
> Our energy cost is about $0.09 kWh
> thanks!!!!

The math on heating water is remarkably simple.  1 BTU / pound / 
degree-difference.  But that is going to be the same for all systems of 
heating water that you want to compare.

What you really need are the various efficiencies and heat losses.  For 
us to help you with that we will need more information.

Some quick things:  Microwave is almost certainly the most efficient way 
to heat water; kettle on the stove is probably least efficient.  Losses 
from tanks are proportional to all of 1) temperature difference (water 
vs environment), 2) surface area of the tank (meaning smaller tanks are 
less efficient) and 3) the u-factor (reciprocal of the R-value (U=1/R)) 
of the insulation.  1 gallon of 190 degree water will provide about 2 
gallons of water at a non-scalding temperature.


Thank You Kindly,

Corwyn

-- 
Topher Belknap
Green Fret Consulting
Kermit didn't know the half of it...
http://www.greenfret.com/
topher at greenfret.com
(207) 882-7652