> I just had a look at my experimental panels on the roof or 
> my shed which is
> at 90 degrees to the house, so better angled, at about 215 
> degrees.
> Theoretically the
> panels are 4 x 15W, so 60Wp.  With the Sun around 20 
> degrees off line and
> about 30 degrees off pitch, I'm seeing no more than about 
> 10W at best!  When
> the Sun
> goes behind a cloud, it drops to zero.  I don't know if 
> it's that the panels
> aren't up to spec. (although they shouldn't be - they were 
> full price) or
> just optimistically
> rated.  Either way, it doesn't bode well for a system like 
> yours working
> here, sadly.

Howard's question extracted from James response.

Are you measuring optimum power or power into a battery.
Maximum power point varies with insolation (sunlevel) and is 
often not optimum for a battery load.
As a rough rule of thumb maximum power occurs when Vloaded 
~= 75%-80% of Voc at a given insolation level. For best 
output people use MPPT (Maximum Power Point Tracking) - a 
fascinating subject in its own right. Simplistically you can 
just dither the panel load and observe the slope of the 
voltage / current curve and drive it in the direction 
required to maximise load current. There are about 3 zillion 
patents related to 'improvements' to that basic technique.


        Russell

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