Hi,

I have an off-grid system at Hua Hin

I have 2 Sofar 20kW inverter, they are set in parallel

17,5 kW battery to each inverter

30 panels Suntech STP545S-C72

Slave inverter have 8 south  panels in serie to MPPT1

7 east panels to MppT2

Master have 8 south panels to MPPT1

and 7 west panels to MPPT2

My system has been working for 3 years and I don't think i can add more batteries

My idea is to add more panels. Because the system needs more power

I heard from a person that I thin know what he say, Longi LR7-72HVD 625-645 is a very good panel

He give me a price of 2,700 baht, I don't know if it is possible to find a better price

He said that when it is a cloudy day, my panel only gives 5%. Longi gives 30%, and even the max output is 15% higher

So my idea is to add 1 to the 8 on carpark

3 to the 7 on east side of roof

9 to the 8 on south side of roof

And 3 to the 7 on west side of roof.

This should be 16 in total

on Slave that should be 9+10

Master 17+10

Now I wounder, as being no expert, should it be ok to do like this

If you mix panels in a string, you will only get the same output as the lowest rated panel.

I replaced all my 340w panels with Rapid 600w bifacial N-type split mono panels, available on Lazada for 1,800bht each Inc delivery and they're really good.

 

https://s.lazada.co.th/s.ZZv0BW

Cheapest at end of month payday sales and monthly double date sales.

24 minutes ago, BritManToo said:

If you mix panels in a string, you will only get the same output as the lowest rated panel.

I replaced all my 340w panels with Rapid 600w bifacial N-type split mono panels, available on Lazada for 1,800bht each Inc delivery and they're really good.

 

https://s.lazada.co.th/s.ZZv0BW

Cheapest at end of month payday sales and monthly double date sales.

So it is like that, you can not mix panels, the least good will be the result

I looked at your link to Lazada. price was 2,640 then I saw promotion and 2 prices 560 and 450. What does it mean?.

Can anybody tell how good this panel is? Better than my old? As good as Longi?

That means I have to change everyone to get better data?!

Just now, Anders Eriksson said:

So it is like that, you can not mix panels, the least good will be the result

I looked at your link to Lazada. price was 2,640 then I saw promotion and 2 prices 560 and 450. What does it mean?.

Can anybody tell how good this panel is? Better than my old? As good as Longi?

That means I have to change everyone to get better data?!

Best prices on Lazada require you using the phone app to order on sale days.

5 hours ago, BritManToo said:

Best prices on Lazada require you using the phone app to order on sale days.

My idea is to do like this. I will take away the 8 panels on south roof and place them  on the other three places.

Then I will try to find a new panel that is very good. Is Longi LR7-72HVD 625-645 the best to find. Please help me to find the best. Important that the panel should be good when weather is cloudy.

Then I will install 17 of that one on south roof, 3, 5 and 9

Then it will be on Slave 9 south + 10 east.

Master 17 south roof +10 west

Someone told me that it have to be the same number of panel on the 2 connections on each inverter

Especially on Master that will be a big difference

7 hours ago, Anders Eriksson said:

Someone told me that it have to be the same number of panel on the 2 connections on each inverter

Especially on Master that will be a big difference

I'd say same total string voltage, not same number of panels.

11 hours ago, Anders Eriksson said:

My idea is to do like this. I will take away the 8 panels on south roof and place them  on the other three places.

Then I will try to find a new panel that is very good. Is Longi LR7-72HVD 625-645 the best to find. Please help me to find the best. Important that the panel should be good when weather is cloudy.

Then I will install 17 of that one on south roof, 3, 5 and 9

Then it will be on Slave 9 south + 10 east.

Master 17 south roof +10 west

Someone told me that it have to be the same number of panel on the 2 connections on each inverter

Especially on Master that will be a big difference

I did something similar - replaced 8 430W panels east facing with 10 Longi 630 W panels, distributed and moved the the 8 430W panels to other strings. 

 

It is true that the 630W panels are hauling a$$ compared to the 430W panels, but your installer is fantasizing if he suggests the larger panels will give a higher percentage of their rated output compared to the smaller panels during cloudy days. 

A word of caution: The solar installers in H-H are exactly that - installers! They are clueless when it comes to optimizing your solar installation or trouble shooting problems - after they turned the switch upon installation, you are pretty much on your own. 

You are doing the right thing - educate yourself, do your research as detailed as your background permits, make your own decisions and have the installer work based on your instructions, not his. 

 

51 minutes ago, mistral53 said:

I did something similar - replaced 8 430W panels east facing with 10 Longi 630 W panels, distributed and moved the the 8 430W panels to other strings. 

 

It is true that the 630W panels are hauling a$$ compared to the 430W panels, but your installer is fantasizing if he suggests the larger panels will give a higher percentage of their rated output compared to the smaller panels during cloudy days. 

A word of caution: The solar installers in H-H are exactly that - installers! They are clueless when it comes to optimizing your solar installation or trouble shooting problems - after they turned the switch upon installation, you are pretty much on your own. 

You are doing the right thing - educate yourself, do your research as detailed as your background permits, make your own decisions and have the installer work based on your instructions, not his. 

 

My 340w  (3,000bht) bog standard panels produced 250w at best dropping to 125w after 3 years.

My Rapid 600w (1,800bht) N-type bifacial split mono panels produce 500w at best.

 

The technology has got better over the past 3 years, and a lot cheaper.

The modern panels start earlier, end later and produce more on cloudy days.

 

On 11/18/2025 at 7:17 AM, BritManToo said:

I'd say same total string voltage, not same number of panels.

I contacted Sofar about this question. They say: PV1 and PV2 can operate at different voltage levels so there should be no problem.

Can you show documentation about "same string voltage"?

30 minutes ago, Anders Eriksson said:

I contacted Sofar about this question. They say: PV1 and PV2 can operate at different voltage levels so there should be no problem.

 

Dual input Sofar inverters employ two independent MPPT controllers which of course allow different input voltages within an operating range.

 

The MPPT controllers have a sweet spot where efficiency is maximised so in theory it would be best to have both strings operating around the sweet spot voltage.

 

The optimal voltage range will be available in the spec.

 

8 minutes ago, Fruit Trader said:

 

Dual input Sofar inverters employ two independent MPPT controllers which of course allow different input voltages within an operating range.

 

The MPPT controllers have a sweet spot where efficiency is maximised so in theory it would be best to have both strings operating around the sweet spot voltage.

 

The optimal voltage range will be available in the spec.

 

I can not see anything  about optimal voltage in manual

8 minutes ago, Anders Eriksson said:

I can not see anything  about optimal voltage in manual

 

Full power MPPT voltage range is the sweet spot. You can operate the inverter outside of this range but its best keep in the window with both strings for as long as possible. 
 

16 hours ago, Fruit Trader said:

 

Full power MPPT voltage range is the sweet spot. You can operate the inverter outside of this range but its best keep in the window with both strings for as long as possible. 
 

Please tell me what this means:

Full power MPPT voltage range

19 minutes ago, Anders Eriksson said:

Please tell me what this means:

Full power MPPT voltage range

 

Exactly what it says - A voltage range where the inverter can deliver its full AC power.
 

11 minutes ago, Fruit Trader said:

 

Exactly what it says - A voltage range where the inverter can deliver its full AC power.
 

Sorry that is to hard to understand for me. You can not describe it  in a way so even not experts can understand?

22 minutes ago, Anders Eriksson said:

Sorry that is to hard to understand for me. You can not describe it  in a way so even not experts can understand?

 

Your solar panels are delivering a voltage to your inverter and this voltage can be adjust through panel selection.  Voltage also fluctuates throughout the day with changing conditions.

 

Sofar are providing a voltage range they call Full power MPPT voltage range  in the spec. This is the range where your inverter will perform at its best.

 

The sweet spot.