add panel to existing solar array

[silversaver]

Hi,

I have 26 panel of Bosch 255W panel running in 2 string of 13 panels with SMA 60000TLUS-12 inverter. I would like to add 1 panel to each string, but the Bosch 255W were unavailable and have to use the Bosch 245W instead. My question is will the new 245W decrease the performace of each array since it has lower Voc and Impp??


Spec:

255W Vmpp 30.51 Impp 8.36 Voc 38.00 Isc 8.92
245W Vmpp 30.11 Impp 8.14 Voc 37.80 Isc 8.72

Thank you

[inetdog]

Him

I have 26 panel of Bosch 255W panel running in 2 string of 13 panels with SMA 60000TLUS-12 inverter. I would like to add 1 panel to each string, but the Bosch 255W were unavailable and have to use the Bosch 245W instead. My question is will the new 245W decrease the performace of each array since it has lower Voc and Impp??


Spec:

255W Vmpp 30.51 Impp 8.36 Voc 38.00 Isc 8.92
245W Vmpp 30.11 Impp 8.14 Voc 37.80 Isc 8.72

Thank you

The voltage difference does not matter since you are increasing each string by adding the same type panel to each.
The current difference is less than 5% (in fact just a bit over 2%). The result is that the output of the original portion of each string will go down very slightly, and that difference will be a lot less than the power you add by putting in the additional panel.

Go for it as long as the new open circuit string voltage, on the coldest possible day of the year, is still within the allowed input range of your inverter.

[silversaver]

panel spec PDF:

[silversaver]

The voltage difference does not matter since you are increasing each string by adding the same type panel to each.
The current difference is less than 5% (in fact just a bit over 2%). The result is that the output of the original portion of each string will go down very slightly, and that difference will be a lot less than the power you add by putting in the additional panel.

Go for it as long as the new open circuit string voltage, on the coldest possible day of the year, is still within the allowed input range of your inverter.

Thank You.

I'm locate in California, so they are still within the range of my inverter. I'm adding 2 X 245W panels and that is about 7.3% increase from a 6.63kW system, but I will lost about 2.25% of original output from a 6.63kW?

[silversaver]

Hi,

I have 26 panel of Bosch 255W panel running in 2 string of 13 panels with SMA 60000TLUS-12 inverter. I would like to add 1 panel to each string, but the Bosch 255W were unavailable and have to use the Bosch 245W instead. My question is will the new 245W decrease the performace of each array since it has lower Voc and Impp??


Spec:

255W Vmpp 30.51 Impp 8.36 Voc 38.00 Isc 8.92
245W Vmpp 30.11 Impp 8.14 Voc 37.80 Isc 8.72

Thank you

The voltage difference does not matter since you are increasing each string by adding the same type panel to each.
The current difference is less than 5% (in fact just a bit over 2%). The result is that the output of the original portion of each string will go down very slightly, and that difference will be a lot less than the power you add by putting in the additional panel.

Go for it as long as the new open circuit string voltage, on the coldest possible day of the year, is still within the allowed input range of your inverter.

So my actual output will be 26 X 30.51(255W) = 396.63 vmp, then 396.63 + 30.11 (245W) = 426.74 vmp, then 426.74 vmp X 8.14 imp (lower current) = 3473.66Wp per string. 2 strings X 3473.66 = 6.947kW total.

My actual increase in only 6947 - 6630 (original 26 x 255W) = 317W instead of 2 X 245W= 490W....., right?

[Wy_White_Wolf]

You got it.

WWW

[inetdog]

So my actual output will be 26 X 30.51(255W) = 396.63 vmp, then 396.63 + 30.11 (245W) = 426.74 vmp, then 426.74 vmp X 8.14 imp (lower current) = 3473.66Wp per string. 2 strings X 3473.66 = 6.947kW total.

My actual increase in only 6947 - 6630 (original 26 x 255W) = 317W instead of 2 X 245W= 490W....., right?

Actually, it is not even as bad as that, since the MPPT circuit on the inverter will play around with the current drain in such as way that actual current through each string will be more than the standalone Imp of the new panel but still somewhat less than the Imp of the original panels.

My off-the-top-of-my head-guess is that you will get at least 400W of additional power when you add the new panels.

And the Vmp of the string will be somewhat less than the sum of the Vmp values of the individual panels combined.
This is one of the type of calculation that is more easily solved by a digital or analog computer (AKA MPPT controller) than by hand.

If the Isc of the new panel were less than the Imp of the old panels, then you would have a real problem and would not get any additional power at all out of adding the new panels. But the difference between Isc and Imp for a silicon panel is so large that if you meet the 5% rule that cannot happen.

[silversaver]

Actually, it is not even as bad as that, since the MPPT circuit on the inverter will play around with the current drain in such as way that actual current through each string will be more than the standalone Imp of the new panel but still somewhat less than the Imp of the original panels.

My off-the-top-of-my head-guess is that you will get at least 400W of additional power when you add the new panels.

And the Vmp of the string will be somewhat less than the sum of the Vmp values of the individual panels combined.
This is one of the type of calculation that is more easily solved by a digital or analog computer (AKA MPPT controller) than by hand.

If the Isc of the new panel were less than the Imp of the old panels, then you would have a real problem and would not get any additional power at all out of adding the new panels. But the difference between Isc and Imp for a silicon panel is so large that if you meet the 5% rule that cannot happen.

So I might get more output than my calculation? I hope my installer has the same panel, but it just hard to find a discontinued panel. I do find the Canadian Solar mono 255W has almost same spec as Bosch, but I guess that will only happen if I need warranty replacement..... oh well, now its matter of doing the math and see if worth of doing it.

Thank you

[inetdog]

So I might get more output than my calculation?

Yes. Your calculation is a lower limit on the available power.

[silversaver]

Here is a good source of reading:

http://www.homepower.com/articles/solar-electricity/design-installation/expanding-your-pv-system

[silversaver]

If I found just a 255W panel instead of two, would it be good to place 255W panel to one string and 245W panel to 2nd string or just use 245W panel at each string?

[inetdog]

If I found just a 255W panel instead of two, would it be good to place 255W panel to one string and 245W panel to 2nd string or just use 245W panel at each string?

Not a big enough difference among those choices to worry about, IMHO.
Go for match of current, don't worry about match of voltage, FWIW.
If the 255W panel costs more, you may not get back the extra cost as extra power. If they cost the same, go ahead and use one 255.

[silversaver]

Thanks for the reply. Just got the pricing today and the cost of adding 2 panels is much higher than I expect. Each additional kWh will cost me $ 0.138 base on 20 yrs production after Fed incentive. No go.... Again, I'm glad I start off a good size solar. I cannot imagine people who try to add more panels later on when looking at their service agreement.

There's no other way to add more panels unless from original installer or you just kiss your warranty good bye.

[Volusiano]

I did a similar thing and got a price quote to increase my system from 11KW to 14KW (planning to add another EV to my household), using the existing inverters, so only 12 panels need to be added to the existing 44 and no new inverter is needed. Original installer agreed that it's technically feasible since there's enough headroom in my existing 2 inverters. However, they gave me a price quote that's only 8% lower than my original installation in terms of cost/KW DC, even though no new inverter is needed. I'm still on the fence on whether I want to add more or not. I'm going to wait until close to the end of the year to see what my whole year usage demand pattern looks like first (system installed last October). But needless to say, I'm disappointed that the additional cost is not substantially lower even though no new inverter is needed, only new panels needed.

But you're right that once you go with an installer, you're stuck with expanding with the same installer due to warranty issue. So they have a leverage and you don't.

[silversaver]

I recently received my SCE POCO bill and learn even I am in standard Domestic plan (4 tiers pricing), SCE still pay me all tiers price... I thought net metering is 1 for 1 trade and they will only pay me tier 1 rate for excess production. I guess if I keep myself in tier 1 and I'll be fine after getting the EV. All about planning...