|One of the little concerns I had in buying the PV
system kit that I did is that it was packaged with 215 watt PV
190 watt micro inverters. It seemed like there was a potential
for the PV panels being able to make more power than the inverter could
handle. Well, the plot below shows an occurrence of just this
happening. I believe that the flat top on the power output curve
that starts about an hour before noon and goes to about an hour after
noon is due to the inverters limiting the output of each PV panel to a
maximum of 199 watts.
I don't feel this is much of a problem because:
The combination of the PV panels being more efficient at low temperatures, very clear skies, and fresh snow all increase the radiation on the PV panels to the point where they can make a little more power than the inverter rating, and the inverter limits the output. The fact that we live at 5000 feet above sea level also increases solar radiation a bit more (about 30 watts per sqm). Hopefully the inverter does not mind running at 199 watts for a few hours under these conditions.
In the couple months we have had the panels, this is the first time I've seen a clear example of a flat top power curve -- so, even here it appears to take an exceptional clear and cold winter day to happen.
For most people, who live at lower elevations and in warmer and less clear sky climates, I don't think this inverter limiting of power output would be seen with these 215 watt panels.
Enphase publishes a list of PV modules that are approved for each inverter. For the 190 watt inverter, this includes PV panels all the way up to 230 watts. If you live in a good sun area with cool winters, you may want to think about not using PV modules that are up at the 230 watt end of the approved module spectrum.
The plot of output below for January 7, 2010 shows the system output for
The picture below shows the individual PV panel output around noon. It shows that each inverter is outputting 199 watts. This is apparently the power level that the Enphase 190 watt inverters are limited to. So, even though the each PV panel is likely capable of putting out a few more watts, the inverter is limiting output to 199 watts per panel.
I would like to hear from anyone who thinks I have this wrong -- Gary...
Update: Enphase has a white paper on this subject that makes a pretty good case for using panels with STC ratings up to about 125% of the inverter rating: Right Sizing PV Modules for Micro Inverters
Feb 2, 2011 -- here is another example that's a little more pronounced than the one above.
For a couple hours on 1/31/11 and 3 hours on 2/1/11 the system output is clipped to 1990 watts. This was a day with very clear skies and a high ambient temperature of +5F -- pretty cold even for Montana. So, it looks like, once in a while on very cold, very clear winter days at our 5000 ft elevation we are going to lose a little power. Just visually, I would guess this is about 0.3 KWH that got chopped off on a day with close to 15 KWH total energy production -- about 2% of this days total.
You can certainly see when the storm ended on 1/31 and the sun came out -- 260 watts to 1990 watts in less than 10 minutes!
While I'm here, I'll just mention that the system has been absolutely trouble free -- its almost boring :)
For a full description of the planning and installation of this system...
Gary January 7, 2010