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We are going to install a PV in our offices. Our actual needs would be satisfied by an installation of about 4kW, so that ~16 panels of 250W. It's going to be a central-inverter, grid-connected installation.

Now, we would probably need more power in a few years. So I'm asking the contractors to quote me a bigger inverter, say, one for 6kW, thinking that if I already have a bigger inverter, then I can just buy more panels when needed, without having to change the inverter itself. I had the idea that I could put a bigger inverter, say, maybe even a 8kW or so, and it will produce what my panels would produce even if it's just half the capacity of the inverter.

But one of the contractors just told me that it wasn't the case. That the inverters "just work" under specific loads, for instance, the 4kW "doesn't start" if it's not feed 3.2kW (or so) from the solar panels. That doesn't make sense to me, specially since for what I have read the inverters just need a certain voltage to start, and that voltage, for instance, is the same for the inverters (in this case, Fronius) from 3kW to 7.5kW, specifically, 260V. This means (I'm no electrical engineer, but I'm no complete layman either) to me that the same power from the panels array is needed to start any of those inverters, so even if the maximum capacity is different, they can start producing as long as the Voltage from the array is 260V.

I'm not talking about putting 4kW worth of panels and an inverter for 20kW or something like that. I'm talking at most of a 10kW inverter, that it's the max amount I could put of solar panels in our roof, and the max one can generate for oneself here in México (or so I understand).

I understand if they are not at their maximum efficiency if not feed under their expected output range, but that they "don't start" doesn't make sense to me.

Am I missing something?

Thanks!

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Better idea: get panels with their own micro-inverters. Then you can add more panels at will in the future. Bonus: micro-inverters are more efficient than a central inverter and will last longer.

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  • Ok, that's another option, although I believe it's more expensive? It's not like I could put a big amount of panels anyway, I have about 70m2 of usable space, which is according to my calcs about 10kW, and about 40 panels at most. May 27 '15 at 1:42
  • Yes, it is more expensive upfront. However, it may well be cheaper over the long term when you take into account the improved efficiency and the likelihood of never needing to replace them. A central inverter is virtually guaranteed to have a lifespan of only 10-15 years.
    – iLikeDirt
    May 27 '15 at 1:57
  • Ok, this is another solution, but I would really like to know an answer to the original question. May 27 '15 at 15:36
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    This answer is just an opinion... I am a solar installer and I have had a similar ratio of problems per job with string inverters and micros. The biggest difference is that if a micro inverter goes out, the homeowner is much less likely to notice as only one panel is affected. If the installer has any integrity, he will fix it anyway. Replacing a micro inverter is much more work than a string. I have a job right now where I need to fix a pretty new enphase micro inverter right in the middle of an array. I will need to remove 6-8 panels to get to it... so much for improved reliability. Jan 26 '16 at 15:48
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You are right, that contractor doesn't know what he is talking about. Most inverters will produce power at low wattages as long as the Vmp of the string (maximum power voltage) doesn't fall below the spec in the data sheet for the inverter. Most inverters are actually more efficient when operated below full load. For example the Sunny boy 11000TL-US will has a peak efficiency at 20% of rated load, or at 2,200W as seen on the data sheet linked above.

You can always check the data sheet for the model you're looking at. Often they'll have efficiency charts like the one linked above. As a rule of thumb, you generally don't want to run an inverter below 8% of its rating, but that depends on the inverter. You also want to remember that the panels will produce less during the morning and evening so I wouldn't really want an array be be much smaller than 40% of the inverter's rating. By these numbers, a 4kW array would be fine on a 10kW inverter as long as your minimum voltage and maximum voltage are in spec. But your inverter may be pretty expensive...

Thanks, Maxfield Solar

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This depends on exactly what equpiment you're using.

My system was designed to have one inverter for each 2 panels, all synchronized to each other and to the power lines so I can do "net metering" rather than having to maintain any kind of local power storage.

Other kits may be configured other ways. Talk with whoever is designing your installation, or if you're doing it yourself get design docs from the manufacturers.

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  • Thanks for your input. We are doing a central-inverter, grid-connected installation. I will modify the question to reflect this. Everything that I have read mentions what I said: the Inverters will work even if the total power from the panels is less than the rated output from the Inverter. But I don't know much about this, and could be misinterpreting things, and the one who knows (the contractor) told me that they don't... which contradicts everything I have read. That's why I'm asking here. May 27 '15 at 1:38

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