When I bought the last five panels, I'd only thought about using four of them as I have to use pairs of panels to make 35V strings for charging 24V batteries. I had thought that I might sell the spare one or wait until I found another to go with it.
But then it dawned on me that I already had 2x 40W Kyocera panels on the garage roof. By connecting these in parallel to make an 80W "panel" and then the spare 80W panel in series, I could use the last BP panel and get rid of the last of the plastic 15W panels.
With this, my system increased marginally to 1,798Wp but probably makes much closer to this figure now as the 15W panels only seemed to put out about 80% of their official rating after about 6 months in the sun. Amorphous panels do degrade as they are first used but makers are supposed to factor this into their rating so that they declare a panel as being 15W after the stabilisation period. Some unscrupulous makers only declare the "Day One" power output though...
With this lot I've been making up to 63% of all my electricity needs each day (well at least 50% for the last three days in a row).The main benefit has been free hot water each day with my automated water heater dump load absorbing all the spare solar power at lunchtime.
I'm not sure I'd want my panels on the house roof as they keep getting covered in dust and so I wash them about once a week with a pressure sprayer. It seems it's not actually just dust but loads of yellow pollen from trees nearby that is settling on them. Bird poo is also a problem as a large seagull one can cover a whole cell and then stop an entire pair of panels from charging the batteries.
Back in April, I did notice some of that volcanic ash collecting on them too. Not a lot but you could see some specks of what looked like burnt paper that were so fine they turned into a grey smudge if you touched them.
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