Justin Richards wrote:
> I am not really sure about insolation or how to calculate it from our
> lat/long and the math looks complicated and appears to deal with a
> surface that is parallel with the ground.

The real issue is not so much the decrease of energy hitting the collector
due to angle as due to weather.  That can vary significantly from place to
place and have nasty worse case values over a few days to a week.  I had to
do some rough calculations like this in college, and I found that airports
keep good weather records that can give you useful expectation of what
fraction of the time the sun is shining in the nearby area.

The loss due to latitude is pretty small, even for a fixed panel.  For the
best yearly average, tilt the panel down from straight up by your latitude.
If you are at 28 deg south, then point the panel down 28 deg from vertical
towards north.  For a pure black body, the power received due to angle
compared to facing directly to the source is the cosine of that angle.
Since the earth is tilted 23.5 deg, the sun will appear to change in height
by +-23.5 deg over the year at noon.  Cosine(23.5deg) =3D 92%, so at noon a=
t
each solstice you loose 8% due to angle alone.  Other times of the year it
will be better.  At the equinoxes at noon the panel will be facing the sun
directly and therefore there is no angle loss.

The loss due to angle during the day is more severe, since it can go to 90
degrees (cosine =3D 0) and beyond at times.  Tracking panels seek to recove=
r
some of that loss.  They do, but also add expense and complexity.  Last tim=
e
I looked into this, it made sense to just get a bigger fixed panel than a
tracking panel for most applications.

That's all just angle and a pure black body.  Solar panels are not black
bodies and are covered with stuff that reflects a larger portion of the
light at low angles.

You can go nuts trying to figure the total efficiency to the last percent,
then get hit with 50% variation over a week due to weather anyway.


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