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Undersized inverter with my new Tesla Solar panels installation
- Thread starter yolostreet
- Start date
nwdiver
Well-Known Member
How much additional energy (kWh/day & kWh/mo) are you getting after adding the second inverter?
Already responded to this earlier. I'll paste again for your convenience
It will be hard to know this for sure until a similar time of the year in 2019, but I would estimate this to be somewhere between 40-60%.
nwdiver
Well-Known Member
Already responded to this earlier. I'll paste again for your convenience
It will be hard to know this for sure until a similar time of the year in 2019, but I would estimate this to be somewhere between 40-60%.
Apologies... missed it...
A little vector math elucidates the performance of your system:
Represent the orientation of a solar array by its unit normal vector N, with N1 and N2 for your two arrays. Represent the sun's location in the sky by the unit vector S. Then the fraction of full insolation on an array is just the dot product, S dot N (as long as that is positive, 0 otherwise). As the dot product is linear in one argument (N) when the other argument is fixed (S), the performance of the joint array is the same as a single array represented by (N1 + N2) (as long as both dot products are positive).
For an array with azimuth A (North = 0 degrees) and Elevation E (Level Panels = 0 degrees), the vector N is given by (cos(90-A)cos(90-E), sin(90-A)cos(90-E),sin(90-E)). Since your two arrays have azimuths 180 degrees apart, the resulting vector sum N1+N2 will be vertical, i.e. just (0,0,2 sin(58)) = (0,0,1.7).
In other words, if you wanted to run your array with a single inverter with two separate MPPT inputs (one for each roof), you could size the inverter as if you had a single array facing straight up of size 1.7 * 3.54 kW = 6.0 kW, using the usual DC / AC ratio practice regarding clipping.
Cheers, Wayne
Excuse the ignorance here, but why does this hold true?
Is it because the panels heat up and can't put out as much power?
nwdiver
Well-Known Member
Yes, the wider the annual temperature band the wider the annual power band. An array with a peak output of 10kW in February may peak at 7kW in August due to heat. Panels lose ~0.2 - 0.5 %/C depending on panel type.
The inverter always controls current, so ISC is not critical (the inverter does not need to draw all available power, thus this under capacity question). As your graphs show, irradiance leads to higher OCV. The voltage swings will definitely be worse in winter which is the worst case for the edge of cloud situation:
Three Levels Of Solar Reliability
Edge-of-Cloud Effect, may not be what you think it is.
nwdiver
Well-Known Member
Here's an interesting real world example. The flatter top lines are voltage the spikey lines are current. Voltage barely budges when the sun comes out... This is a cold overcast day in February.
Sure, if the inverter is running, the panel never gets to OCV. Rather, the inverter does peak power tracking where the panel stays near the same voltage as it varies the current/ power as the brightness changes. The damage occurs if the inverter is turned off/on while the edge effect is happening. The lower right section of the graph you posted.
nwdiver
Well-Known Member
My only point is that when sizing an array for VOC insolation or cloud edge effect isn't even a factor... only temperature.
Yah, temperature has a greater effect on Voc than insolation.
Article on general temperature effects for anyone interested The Highs and Lows of Photovoltaic System Calculations
ai4px
Wes
I am also supposed to get a 12.8 system but they want to install the same as yours. 7.6 inverter. They told me I could go To a higher sister but that they wouldn’t add another inverter. I may just forget it and go with a local company.
I would read some of the rather extensive discussions here about efficiency and in particular about the putative losses for having an "undersized" inverter. For most of the owners here, the power (kWh/day) gain from expanding to a larger inverter from a 7.6kW inverter on a 12.8kW system has been on the order of 1-2% initially, and probably less than that over the system lifetime, I.e. unlikely to be worth the expense.
All the best,
BG
It may not be worth it but the ratio is 1.7 which is a little high. After talking with Tesla again I was told that even if I went with a 14.4 system I might not get a second inverter. I’m not sure that could really be the case unless all the panels were facing north. ( they aren’t)
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