There was a post on Edie recently that stated that the Met Office's solar array at Exeter had over-performed last year compared with expectations. This is despite 2012 being one of the wettest and least sunny years on record. My immediate thought was this all begs the question of how the solar array output forecast had been done in the first place.
Being a bit of a nerd I ran a quick assessment before we acquired and had installed our little (about 100 times smaller than the Met Office) array. Because I was looking to test the economic viability of my scheme I was cautious in my outlook. For instance, I assumed that I only got the manufacturer's minimum performance guarantee level. That was pretty certainly going to be worse than year 1 performance. I also rounded the array's orientation and pitch to the nearest 10 degrees in a detrimental direction. Surprise, surprise! My 2012 performance figure outstrips my "prediction" (2570 kWh actual vs 2157 kWh predicted). I'm pleased, of course, because this suggests that payback will probably be quicker than my model forecast but I really should not be surprised - I bent the inputs to make such an outcome likely. Did the Met Office do the same?
Incidentally, a number of friends have used my model (it's in Excel) to assess their projects and I can make it available on a "no prejudice" basis if you drop me an email.
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