[amsat-bb] Re: Solar Power (I was wrong)

Dave Larsen PhD doc at volcano.net
Fri Oct 2 12:16:29 PDT 2009

I think you are also forgot the price of a  Building permit  - in this 
county 1-2K


----- Original Message ----- 
From: "Robert Bruninga" <bruninga at usna.edu>
To: "'Greg D.'" <ko6th_greg at hotmail.com>; <ptrowe at yahoo.com>; 
<amsat-bb at amsat.org>; <wb3jfs at cox.net>
Sent: Friday, October 02, 2009 11:27 AM
Subject: [amsat-bb] Re: Solar Power (I was wrong)

> ... I have... a roof angle exposure that is
> not solar-friendly...

That was another error I had made in my situation.

Again, there is a BIG difference between solar design for
stand-alone battery systems (must point south, optimum angle,
etc) and grid-tie systems that can be anything from SE to SW and
even FLAT and still be only a few percent off peak!.

The economics are entirely different.

The remote/battery system, MUST provide a minimum power on the
worst winter days and weather of the year.  It MUST be optimized
for winter.

On the other hand, the grid tie system only has to have a good
AVERAGE power averaged over a year.  And you can make more money
in a month of summer sun making money at HIGH payback rates than
you can get all winter (3 months).

Even a FLAT array makes more power in the 3 summer months than
the optimum tilt angle array does.

And lastly, anything pointed SE to S to SW is about the same for
a grid-tie system.  You lose at most about 5% SE or SW compared
to South.  Again, the reason is not obvious.  But any tilted
array is only going to see the sun for 180 degrees of path
across the sky.  Even the ideal South facing tilted array does
not see the morning sun nor the evening sun in the summer when
you are getting your most payback..  It only sees the midle 6
hours.  Since the  sun is up much longer than that most seasons,
then getting those 6 hours averaged before noon (SE) or getting
them in the afternoonn (SW) makes little difference for a
grid-tie array, though, I'd favor SW, since peak electric rates
apply longer in the afternoon than in the morning.  So you want
to maximize your power when electricity rates are highest.

Again, I am sharing this off topic with everyone, because I too
learned that my thinking was all wrong based on my previous
experiences with stand-alone power systems and that a
grid-tie-system has completely diffeerent economics to my normal

You can play with all the angles and directions for grid-tie
systems on-line with the solar energy calculator here:

Yes, SOUTH with a latitude tilt is best... But based on annual
 Southeast only lost 5%
 Southwest only lost 5%
 Droping the tilt to the angle of my roof 25 deg only lost 1%!
 Droping the tilt to FLAT on the ground only lost 14%
 (but if I tilt them up to 45deg Sept to April) I GAIN 20%!  And
that is a +5% over optimum south.


On the other hand, ANY shade will significantlly cut into your
power budget.

Bob, Wb4APR

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