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From: morrison@cs.uiuc.edu (Vance Morrison)
Newsgroups: sci.energy,sci.electronics,sci.physics
Subject: Re: solar cells
Message-ID: <1991Jan1.184225.26995@ux1.cso.uiuc.edu>
Date: 1 Jan 91 18:42:25 GMT
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wrf@mab.ecse.rpi.edu (Wm Randolph Franklin) writes:

>The reason I  ask is that  many "efficient" devices make  no sense in my
>home when interest  is included.  E.g.,  I take years to burn  out  a $1
>100W power 1700 lumen  1000 hour bulb.  Some haven't  burnt out since we
>bought the house 8 years  ago.   A  $10 fluorescent bulb  would  have to
>contain a nuclear  power source and  provide the   light free   to be as
>cheap if money costs 12%.

I agree that in gerneral the cost of money is NOT taken into account
for many concervation systems.  On the other hand, in the case of 
fluorescent bulbs, I think they DO in fact pay for themselves even when
the cost of money is taken into account.

Lets take your example.  Lets also assume that both bulbs have a 10 year
lifetime (if anything this makes my analysis concervative).  Lets also
assume that interest is 12% and that the light is used 3 hours a day 
(conservative for a living room light), and that electricity costs
$.07/Kw-hr (thats what I pay, your rates may vary.).   

Lets assume that I have $10 to spend.  I could

1) By a flourescent bulb and use it for 10 years.

2) By a incandecent for $1 and save $9 in the bank at %12 interest 
   compounded daily for an effective rate of 12.7% for 10 years.


In both cases I have used the bulb for 10950 hours over 10 years.  
The incandecent uses 75W and the flourecent uses 18W.  

In the first case I must pay (10950*18/1000)*.07 = $13.80 of electricity

In the second case I must pay (10950*75/1000)*.07 = $57.49 of electricity
But I also have the money I made in the bank 9*.127*10+9 = $20.43 Thus
my 'true' cost is 57.49-20.43 = $37.06.  

Thus I have still saved (made) $23.26 by using the flourescent, even
with interest taken into account.  

This analysis does not take into account the fact the value of the money
(interest) for the electricity payments, however, adding this correction
will only help the case of using flourescents.  

--------------------------------------------------

Now in this analysis I used the original number, I would have used
different numbers.  I can buy flourecent adapters for $12 not $10 (but
replacements for just the bulb for $8).  Also, since I don't take out
loans for things under $100, the real choice is between saving that
money or 'investing' it in a flourescent.  Thus I would have used
%8 interest (since thats what I can get in a CD).  It really doesn't
matter because the results point the same way.  

The only 'dubious' assumption I have made is assuming 3 hour use.
Certainly not every light in your house sees this much use, but certainly
some do, so those are the ones you should replace. 

Vance