Hi Digit,
Yes, the inverters are the weakest link in most PV systems. As long as the panel (which is basically just a plastic laminate, a layer of silicium solar cells, a layer of glass and an aluminium frame) stays watertight, it will continue to work; it has no electronic nor any mechanical components. The inverters on the other hand contain a lot of electronics and - as we all have experienced - electronic devices will fail sooner or later.
I have probably been quite unlucky, but here's my track record of the past 5 years:
- Philips recalled their inverters *twice* because of - as they said - a fire hazard in the electronic circuit. I happened to have 5 of these inverters.
- Many early systems were equipped with NKF OK4E inverters. These inverters were mounted *outside* on the back of the panel. In other words: they supplied 230 AC directly from the panel on the roof into the house. These inverters turned out to be quite unstable. Reports mentioned failure rates of over 50% after a few years. NKF and solar panel suppliers (mainly Eneco, Nuon and the like) decided for a large scale replacement operation (> 10.000 installations!), having an installer replace the panel mounted inverters for a single Mastervolt Soladin 600 string inverter inside the house. I happened to have 8 panels with NKF inverters; they got replaced by 2 Mastervolts. Interesting detail: the NKF inverters had a "KEMA-keur" printed on their label, but they'd never been tested by KEMA
.
- I have 5 Exendis Gridfit inverters. One of these had a strange alternating behavior: if solar radiation increased to a certain level, it would continuously reset itself. I had it replaced. The replacement inverter broke down (blown fuse?) within a month after replacement. I had it replaced a second time.
- I have 12 BP solar panels. They turned out to have a production defect that could cause the frame to become "powered", due to using the wrong sealant. I'm currently awaiting replacement panels (these panels are currently disconnected).
Although I must say that suppliers and manufacturers have been very helpful in getting these problems solved, at no cost at all, often paying for "lost" kWhs and sometimes even out of warranty, it's been quite a hassle to deinstall/install the failing components and arrange replacements. This equipment was definitely not at the consumer-grade level back in 2002/2003.
Don't let this story scare you though: I think things are better nowadays. Note that I have experienced only a single real malfuntion; the other incidents were preventive replacements and/or product call backs. And given the fact that almost all of my inverters have been replaced, they should last for another 10 years without any problems, right?
. That's at least what is currently seen as a reasonable lifespan of an inverter.
Inverters with a good reputation are:
- SMA Sunnyboy (several, also for larger systems)
- Mastervolt Soladin 600
- Stecca 300/500
- Exendis Gridfit
As far as your questions about the needed surface is concerned: it all depends on the way you place the panels. Usually, you would try to position the panels to the South, at an angle of 36 degrees; that would give you the optimal annual revenue. On a flat roof, this is easily done using PV mounts. However, if you put them at an angle, you need a certain amount of space between rows of panels, because the panels would otherwise be in the shadow of the rows in front. In general, given a flat roof of 100 m^2 (like you have), you would be able to place around 40 to 50 m^2 of solar panels. That would be around average 40 panels.
In my case, the situation is a little different. As I already had the panels, my intention was to place as many panels as possible, even if that meant that the position of each individual panel would not be optimal (I'd rather place 40 panels at 75% efficiency than 20 panels at 100% efficiency). Hence the more or less "flat" installation in line with our roof. This would in theory still be approximately 75% efficient. We now have 42 panels installed, taking up roughly 50 square meters (the main roof has 32 panels and is approximately 40 square meters).
Lennart
