Check my math on mixing panels?

[Fffrank]

I have (4) 240w panels on my RV with the following specs:

    Open circuit voltage (VOC): 37.2 V
    Max power voltage (VMP): 30.4 V
    Short circuit current (ISC): 8.37 A
    Max power current:  7.89 A

This string is wired in 2S2P


My plan is to add an additional (3) panels that are 195w each with these specs:

    Open circuit voltage (VOC): 24.5 V
    Max power voltage (VMP): 20.2 V
    Short circuit current (ISC): 10.14 A
    Max power current:  9.65 A

I was going to wire these panels in series and then all (3) strings in parallel with the larger panels.

Using this online panel calculator I get:

All identical panels wired in SERIES, then all series strings wired in PARALLEL:
    Max power output: 1547.36W
    Max power voltage (Vmp): 60.8V
    Max power current (Imp): 25.450000000000003A
    Loss: 0.25%

I'm using a Victron 150/70 so a 60.8v / 25A maximum is perfect. But.... here's where my math breaks down. The 4x240w panels are currently putting out 70-72v rather than the 60.8 that the calculator tells me. What am I doing wrong and is my plan still good for wiring?

 

[sunshine_eggo]

I have (4) 240w panels on my RV with the following specs:

    Open circuit voltage (VOC): 37.2 V
    Max power voltage (VMP): 30.4 V
    Short circuit current (ISC): 8.37 A
    Max power current:  7.89 A

This string is wired in 2S2P


My plan is to add an additional (3) panels that are 195w each with these specs:

    Open circuit voltage (VOC): 24.5 V
    Max power voltage (VMP): 20.2 V
    Short circuit current (ISC): 10.14 A
    Max power current:  9.65 A

I was going to wire these panels in series and then all (3) strings in parallel with the larger panels.

Using this online panel calculator I get:

All identical panels wired in SERIES, then all series strings wired in PARALLEL:
    Max power output: 1547.36W
    Max power voltage (Vmp): 60.8V
    Max power current (Imp): 25.450000000000003A
    Loss: 0.25%

I'm using a Victron 150/70 so a 60.8v / 25A maximum is perfect. But.... here's where my math breaks down. The 4x240w panels are currently putting out 70-72v rather than the 60.8 that the calculator tells me. What am I doing wrong and is my plan still good for wiring?

View attachment 258582

Pretty good match.

String Vmp within 10%, and you should have no issues.

Your string Vmp deviation is < 1%, and most panels have a 3%-ish variation from spec, so it's just noise.

Vmax is Vmax. It's the maximum observed voltage that day, not your operating voltage.

If you're in absorption, V will be above Vmp as the full output isn't being pulled from the panels, so voltage rises.

I'm more worried about your paltry Pmax numbers.

What does your trends tab say for PV voltage and current?

 

[Fffrank]

I'm more worried about your paltry Pmax numbers.

What does your trends tab say for PV voltage and current?

Thanks for validating that I wasn't totally off here.

Here's my last 90 days. But I wouldn't expect that I have needed much/any production as it has been plugged in that entire time. I'm also at the 46th parallel and they are flat mounted so this will never be a system that performs particularly well.

 

[sunshine_eggo]

Thanks for validating that I wasn't totally off here.

Here's my last 90 days.

The MPPT only stores 46 days, and you are zoomed to 60, not 90. The goal was to establish what your typical Vmp is, and if you can get any useful detail out of 46 days of very coarse data, you're better at this than I am.

But I wouldn't expect that I have needed much/any production as it has been plugged in that entire time. I'm also at the 46th parallel and they are flat mounted so this will never be a system that performs particularly well.

If you NEVER get max power out of the array, PV voltage will always be higher than Vmp. If the system is lightly utilized, it's likely that the batteries/loads never need much topping off, so you're always higher than Vmp.

Wait until noon and then load the system with several hundred Watts. Then evaluated the data.

 

[Fffrank]

Is this closer to what I should expect? Ran the microwave for a few minutes until I saw the victron flip into BULK.

 

[sunshine_eggo]

It seemshigh, but if it's very cold where you are, Vmp can be higher than spec.

 

[Fffrank]

It seemshigh, but if it's very cold where you are, Vmp can be higher than spec.

Currently 15F / -9.5C

 

[SouthernSolar]

I'm running a bigger mis match than that with no problems. 3 strings at ~95 volts verses 1 at~ 110. I lose a little power, but I'm over paneled anyways.

 

[sunshine_eggo]

Currently 15F / -9.5C

Yeah, you should definitely be above Vmp. Probably about 8% high - 65V is probably about as low as you'll get in those conditions.