proper string inverter sizing

[ButchDeal]

I guess I don't understand why SMA even makes a 7.7kW inverter if it can only handle 22 panels?

It can handle more modules, you just need a combiner box with fuses.

[charles2]

I guess I don't understand why SMA even makes a 7.7kW inverter if it can only handle 22 panels?

[sensij]

They have a 46.3 Voc. (Datasheet)

14 * 46.3 = 648 V. Way too much.
13 * 46.3 = 602 V. Too much.

Assuming the temp correction used in the previous post was correct, the maximum number of those panels allowed in a string on that inverter is 11.

[charles2]

Would you run thru the minimum and maximum string sizing calculations for Suniva OPT325-72-4-100 modules and the SMA Sunny Boy SB 7700TL-US-22 inverter? I'm hoping it's OK to put 13 of those modules on one string and 14 on the other.

[sensij]

So there is no clipping on the low end, and you cannot destroy an undersized inverter?

I'm sure you can come up with extreme cases that can have a bad result. However, when the inverter is modestly oversized or undersized... for example, when the ratio of the DC panel rating to AC inverter output rating is between 0.67 - 1.35, different ratios are just going to have small differences in efficiency.

You can see a typical efficiency curve for an SMA inverter in their datasheet. As long as the inverter is putting out at least 20% of its rating, the efficiency is pretty good, and it doesn't really fall off the cliff until you get down to the 5% range.

[charles2]

So there is no clipping on the low end, and you cannot destroy an undersized inverter?

[sensij]

Thanks, that's good info.

Tell me, what happens when an inverter is oversized or undersized? I'm guessing that an oversized inverter clips array output when it is weak, and an undersized inverter either is destroyed or clips excessive array output. Am I correct?

If undersized relative to the DC power available from the panels, the inverter will limit its output to something close to its AC rating. On the other end, except at the extremes, for the most part having an inverter too big is just money that you could have spent on something else.

[charles2]

Thanks, that's good info.

Tell me, what happens when an inverter is oversized or undersized? I'm guessing that an oversized inverter clips array output when it is weak, and an undersized inverter either is destroyed or clips excessive array output. Am I correct?

[sensij]

SMA has an online design tool and a downloadable spreadsheet that may help.

If I were designing it, I would do something like this, consistent with what you'd get from one of the design tools (I checked).

If your panels are these, look at the following:

@STC
Isc = 8.80 A
Voc = 45.0 V
Vmp = 37.0 V
Temp coef of Voc = -0.30%/deg C

A string of 13 will have a Voc of 585 Vdc and a Vmp of 481 Vdc @ STC.
A string of 10 will have a Voc of 450 Vdc and a Vmp of 370 Vdc @ STC.

Athens, GA should use -11 deg C as the minimum design temp. [(-11) - 25] * -0.3%/deg C = 10.8%.

Temp corrected Voc for a string of 13 = 585 * 1.108 = 648 V.
Temp corrected Voc for a string of 10 = 450 * 1.108 = 499 V.

This series of inverter has a maximum Voc of 600 V. So, 13 panel string is no good, but the 10 panel string is OK. 12 panel string would be OK too, if you can drop a panel from the big array.

If you can't drop a panel, splitting it into a 6 and 7 panel string is a bad idea, they need to be equal to minimize losses. Therefore, add a panel and make it two strings of 7. VOC @ STC of a 7 panel string is 350 V, safely above the minimum string voltage of 125 V.

So, now you either have 22 or 24 panels. 24 * 310 = 7440 W, 22 * 310 = 6820 W. The SB6000TL-US-22 would be a good choice for either of those combinations, especially since the two arrays won't hit peak power at the same time due to the difference in their orientation. It has two MPPT inputs so put each sub-array on its own input and you'll be fine.

[charles2]

For my 2 Renesola subarrays composed of 310W modules, I want to use a single SMA Sunny Boy TL-US-22 series inverter that has the "Secure Power Supply" feature. I will have 13 modules facing mostly south and 10 modules facing mostly west. What is the proper size for the inverter?