Westward Ho!

I've finally got around to processing some of the data from our new garage roof mounted solar PV.

Because the house PV is facing south, and the garage is facing west, we expected the garage PV to start later, and finish later.  On the nice sunny 12th of May, that's exactly what we see:

blue = half the south facing house PV; orange = west facing garage PV; x-axis times in BST; y-axis production in Watts

We're getting nearly half as much again solar production from the new panels (the blue line is showing only one of the two banks of panels for house PV production, to make comparisons easier).  The new system peaks later, so we are extending the day to some degree.  But, the two systems actually start and stop generating at about the same times.  On 12th May where we are, sunrise was 5:07 BST, sunset was 20:44 BST.  So they both start generating just after sunrise (despite the garage panels facing west, directly away from the rising sun), and both stop just before sunset, albeit one generating much more than the other around these times.  

This is due to the panels generating a small amount from diffuse sunlight reflecting from the rest of the sky, in addition to generating the bulk from direct sunlight.

The effect of diffuse sunlight generation is even clearer on 16th May:

blue = half the south facing house PV; orange = west facing garage PV; x-axis times in BST; y-axis production in Watts

This was a cloudy day: note the different y-axis scale, here maximum production per bank of panels was around 1.4kW, as opposed to the 3.5kW on the sunny day.  On this day, sunrise was 5:01 BST, sunset was 20:55 BST, essentially the same as on the 12th.  There was no direct sunlight: it was cloudy all day.

Now the two curves are essentially identical, just tracking the thinning and thickening clouds: the different orientations of the panels has practically no effect.

More charts coming later!