> Some time ago it was mentioned that opening the door on a 
> fridge was a major energy expense. That seemed somehow wrong 
> to me so I finally did some calculations but I frequently 
> mess such stuff up so I am asking for a reality check from 
> the rest of us.
> 
> The data:
> sp heat of air  1 J/(gram degK)  - (wikipedia) density air 
> .0745lb/cuft -  (don't remember wherefrom)  = 33 g /cuft 
> (how's that for mixed units? :-)  )
> 
> calcs:
> so we get 33 J/cu ft deg C   = 33Wsec / cuft deg C
> 
> my fridge is about 12 cuft  (33" x 23" x 26") so I get  396 
> Wsec/deg C, drop the temp from 25 to 0C gives 25 x 396 = 9900 
> Wsec per cool down lets make up, say, 20% efficient gives 
> 49,500Wsec  ~~50,000Wsec so, 50kWsec/60= 833 KWmin = 13.8 
> KWhr  at $.10/KWHr = $1.38 / cooldown Can this be so?  I'm 
> never gonna open that sucker again as long as I live!! At 
> least not till I need the next cold beer.


That number is a tad large.  I'm too lazy to go back & check, but I think it
was under 1/4 of a cent per opening last time we worked it it out.

You could always buy a watt meter and put that on the fridge for a few days.
I was always a bit suspicious about the 'major energy loss' claims too.  I
worked out my fridge cost a few cents per day to run, so little that the
payback period if I brought a new one was over 20 years.  IIRC, it used
~100W when running, and ran 2-3 hours per day.

That said, the bloke with the chest freezer is right, a chest freezer won't
loose that cold air when the door opens, so it'll use less power.  Probably
not worth it for the average person, but he was trying to squeeze every watt
posible out of his solar system or something.  Supermarkets know this, they
don't even bother with doors (lids) for their freezers.

To claims the savings that he did, I think he just had a crap fridge to
start with.

Tony


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