proper string inverter sizing

[sensij]

I don't think the spreadsheet is accounting for the difference in MPPT range between the different models of inverter.

4550 / 4000 = 1.14... this is a good loading ratio. The panels will typically produce less than their rating, so you won't really be missing out on any power.

You might actually be better off going for a pair of the SB3800TL, with a 1.20 ratio. You can use a 40 A breaker to protect two of those, which in some cases will help avoid the expense of a panel upgrade, in addition to the savings from going with the smaller inverter.

You can use PVWatts and vary the DC-AC ratio in the Advanced Parameters to get a feel for how much energy you might miss out on by undersizing the inverter. Until the ratio gets to 1.25 or more, with a good array orientation, it usually isn't much.

[charles2]

Use two SMA inverters (SB4000TL-US-22), which have a 175 V MPPT minimum and would be fine with strings of 6. In that case, you would have 4 mppt inputs. On inverter 1, input 1, you could have 6 panels, inverter 1 input 2 7 panels, inverter 2 input 1 7 panels, inverter 2 input 2 7 panels.

Since 325*14 = 4550, wouldn't you want to go with 5000TL-US-22 rather than 4000TL-US-22?

Also, I'm very confused by SMA's string sizing spreadsheet at SunnyBoyTLUS-22_QDR_v1.10.xlsx. It seems to indicate that the modules per string can vary from 5 to 13.

[sensij]

This is a hot Phoenix site with 6/12 pitch and 60-cell panels with Vmp= 8.53Vdc so there is never a time when both the East and West sides peak out. There may me instances when the inverter will have to throttle back a bit, but this design comes straight from the advice of a SMA app. engr and it checks out fine on their string sizer.

Thanks for the response. The 6:12 pitch helps. I'm glad to hear your ability to understand the system and propose a creative design was actually rewarded with some business!

[solarix]

This is a hot Phoenix site with 6/12 pitch and 60-cell panels with Vmp= 8.53Vdc so there is never a time when both the East and West sides peak out. There may me instances when the inverter will have to throttle back a bit, but this design comes straight from the advice of a SMA app. engr and it checks out fine on their string sizer.

[sensij]

I'm doing a SB7700 right now that has 12 260W panels facing south and 2 strings of 8 facing west and another string of 8 facing west. It takes a DC combiner to put all three strings of 8 in parallel on one inverter input, but the alternative was 2 inverters. Putting the east and west strings on the same input is a good trick because they will lose very little productivity and still allow doing this roof with one inverter. My quote won out over the competition because of this.

Interesting. Clearly the Isc of the three 8 panel strings will exceed the 19 A Isc limit, and even with E/W orientation and knowing the two faces won't peak simultaneously, I'd be worried that three strings would exceed the 18 A input limit. Are you just counting on the inverter moving away from MPP and limiting the power in that case? If you don't mind sharing, what zip, tilt, and azimuth? I'm curious to see what PVWatts/SAM would show for this.

[solarix]

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

I'm doing a SB7700 right now that has 12 260W panels facing south and 2 strings of 8 facing west and another string of 8 facing west. It takes a DC combiner to put all three strings of 8 in parallel on one inverter input, but the alternative was 2 inverters. Putting the east and west strings on the same input is a good trick because they will lose very little productivity and still allow doing this roof with one inverter. My quote won out over the competition because of this.
However, your point is well taken that using large panels on string inverters is again becoming pretty restrictive. On the older inverters, with narrow voltage operating ranges, there was a time when the panels sizes got up around 200W that strings had to be 10, 11, or 12 panels - period. Now with the >300W panels, we're running into these problems again... We find that the mainstream panels sizes of ~260W are better in that they have less string sizing problems, give us more flexibility fitting them onto most roofs, are about the limit that can be handled by one installer carrying them, and are better value. Of a course the high efficiency panels get around some of these problems but cost way more.

[charles2]

Did you read my previous response?

I did but guess I'm having trouble paying attention. Thanks for the other options.

[sensij]

Does anyone agree or disagree that what I've proposed is the best solution?

Did you read my previous response? Min string length is 8 for that inverter. Options I see:

1) Use SolarEdge or microinverters

2) Use two SMA inverters (SB4000TL-US-22), which have a 175 V MPPT minimum and would be fine with strings of 6. In that case, you would have 4 mppt inputs. On inverter 1, input 1, you could have 6 panels, inverter 1 input 2 7 panels, inverter 2 input 1 7 panels, inverter 2 input 2 7 panels.

3) Use a Fronius Primo 7.6 inverter. It has a min MPPT of 150 V, or maybe as low as 80 V. Then split into two strings of 6 into one input, and two strings of 7 into the other input, dropping the odd panel as you suggested.

4) Use a different panel, preferably 60 cell, that operates at a lower voltage as has more flexibility.

[charles2]

2 strings of 6 panels each on one slope and 2 strings of 7 panels each on the other slope?

Does anyone agree or disagree that what I've proposed is the best solution?

[charles2]

I think the problem is 13 panels are on one roof face, and 14 are on the other. 60 cell panels would have more flexibility, but may not use the available space on the roof as efficiently.

You are exactly correct. So what's the best I can do?

What if I drop the odd panel, so that I'll have 2 strings of 6 panels each on one slope and 2 strings of 7 panels each on the other slope?

[ButchDeal]

I think the problem is 13 panels are on one roof face, and 14 are on the other. Splitting them into parallel strings doesn't work great. A string of 6 * 37.0 Vmp = 222 Vmp, which is too low for this inverter (low MPPT range is 270 V). With the 72 cell panels the OP picked out, it looks like strings should be in lengths of 8 to 11. 60 cell panels would have more flexibility, but may not use the available space on the roof as efficiently. The inverter operates below the MPPT range down to 125 Vdc, but I'm not sure of the consequences for being outside of the range.

One of the many reasons why I have change our company base inverter to SolarEdge. This design is trivial with SolarEdge and optimal performance for the layout.

[sensij]

I think the problem is 13 panels are on one roof face, and 14 are on the other. Splitting them into parallel strings doesn't work great. A string of 6 * 37.0 Vmp = 222 Vmp, which is too low for this inverter (low MPPT range is 270 V). With the 72 cell panels the OP picked out, it looks like strings should be in lengths of 8 to 11. 60 cell panels would have more flexibility, but may not use the available space on the roof as efficiently. The inverter operates below the MPPT range down to 125 Vdc, but I'm not sure of the consequences for being outside of the range.

[inetdog]

So what's the best way to set up these 27 Suniva panels with the 7700TL-US-22? Should I drop a panel to get an even number, or what?

You do not need an even number. You just make one string with an odd number (like 9) leaving an even number (18) which you then divide by two.
The -22 model has two MPPT inputs, so you do not need a fused combiner. Just put one string on one input and the other two strings in parallel on the other input.
If you had 26 or 28 the situation would not be any better or worse. With 26 you can go for one string of 8 and two strings of 9 or one string of 10 and two strings of 8 if you want to distribute the power more equally between the two MPPT inputs.

[charles2]

The design constraints for strings are tighter than they are for microinverters or optimizer systems, which is one of their disadvantages, but workable designs often exist.

So what's the best way to set up these 27 Suniva panels with the 7700TL-US-22? Should I drop a panel to get an even number, or what?

[sensij]

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

You don't even need the fuses or a J-box, in some cases. For example, a 28 panel system using the 72 cell panels mentioned above might have two strings of 9 combined in parallel on one MPPT input (using MC4 Y-connectors), and a 10 panel string on the other input. Fuses aren't required until at least three strings are being combined on a single input.

The design constraints for strings are tighter than they are for microinverters or optimizer systems, which is one of their disadvantages, but workable designs often exist.