How much do PV voltage change

[x4dow]

If a solar panel has 41 VoC and 34 Vmp what sort of voltages do you get of it through the day?

I ask because I'm building a system that will likely be 160VoC/136Vmp (4 of above in series) on a controller with mppt range of 120-450v

Is it safe to say the panels will reliably output over 30v each?

In other words, is it the current that goes up and down and Sun comes and goes and voltage stays relatively steady? Or does voltage drop to the 20s with shading/clouds etc?

 

[Forbisher]

In other words, is it the current that goes up and down and Sun comes and goes and voltage stays relatively steady?

You got it.
You will get very close to Vmp when you have sun and load.

 

[x4dow]

You got it.
You will get very close to Vmp when you have sun and load.

Thank

Very close /above vmp I assume? Not under

 

[Forbisher]

Very close /above vmp I assume? Not under

Voc and Vmp will change with temp.
Panel output is rated at 77°F
Colder temps will raise voltage.
Hotter temps will lower voltage.

 

[Hedges]

Voltages only vary a bit under uniform illumination as brightness and temperature vary.

But if a shadow covers a single PV cell, even though Voc will be virtually unchanged, Vmp will drop according to one diode-bypassed section of PV panel (typically 1/3 or 1/2 of panel voltage).
When current is drawn, that section of panel puts out nothing so only Vmp and Imp of remaining sections of panels contribute.
If 1/2 the area of one cell is shaded, then you get 1/2 Imp at Vmp of the string. MPPT has to be smart enough to pull voltage lower and get higher current of 1.0 Imp, bypassing that section.

Similar deal if two strings in parallel and some panels of one string shaded. There may be two peaks in the watts/volts curve and MPPT ideally finds the higher one, but that depends on firmware.

Very close /above vmp I assume? Not under

Definitely can be under Vmp. Vmp was determined with laboratory conditions, flashed with 1 "sun" but panel at 25 degrees C panel temperature.
Real world conditions are typically warmer (could be colder) and typically less light (could be more).

So voltage delivering most power could be higher or lower than Vmp. You ought to design your system to work above the minimum MPPT voltage range of your controller; otherwise, it will sit at that minimum and get less than maximum power from panel.

136Vmp for 4 panels, 120V minimum mppt.
I think that will usually work, look up the temperature coefficient of Vmp in data sheet of panel.
It is less clearly documented how Vmp changes with illumination. Data sheet may plot curves, but if shows only as I/V not W/V, difficult to tell.

If a shadow ever falls on a panel, power output may drop to zero. I've done that, where I reconfigured some 12V panels for 16s2p instead of 24s1p. Didn't take much shade to stop producing.

You'd be better off with 5 panels in series.

 

[x4dow]

Voltages only vary a bit under uniform illumination as brightness and temperature vary.

But if a shadow covers a single PV cell, even though Voc will be virtually unchanged, Vmp will drop according to one diode-bypassed section of PV panel (typically 1/3 or 1/2 of panel voltage).
When current is drawn, that section of panel puts out nothing so only Vmp and Imp of remaining sections of panels contribute.
If 1/2 the area of one cell is shaded, then you get 1/2 Imp at Vmp of the string. MPPT has to be smart enough to pull voltage lower and get higher current of 1.0 Imp, bypassing that section.

Similar deal if two strings in parallel and some panels of one string shaded. There may be two peaks in the watts/volts curve and MPPT ideally finds the higher one, but that depends on firmware.



Definitely can be under Vmp. Vmp was determined with laboratory conditions, flashed with 1 "sun" but panel at 25 degrees C panel temperature.
Real world conditions are typically warmer (could be colder) and typically less light (could be more).

So voltage delivering most power could be higher or lower than Vmp. You ought to design your system to work above the minimum MPPT voltage range of your controller; otherwise, it will sit at that minimum and get less than maximum power from panel.

136Vmp for 4 panels, 120V minimum mppt.
I think that will usually work, look up the temperature coefficient of Vmp in data sheet of panel.
It is less clearly documented how Vmp changes with illumination. Data sheet may plot curves, but if shows only as I/V not W/V, difficult to tell.

If a shadow ever falls on a panel, power output may drop to zero. I've done that, where I reconfigured some 12V panels for 16s2p instead of 24s1p. Didn't take much shade to stop producing.

You'd be better off with 5 panels in series.

im trying to get a setup that fits into 2.4x3.6m. so might have to look into (4) 72cell panels or 5 small 250-300w 60 cell ones

 

[Hedges]

Check also if MPPT voltage parameters can be adjusted.

But, for some architecture, required PV voltage is related to the voltage rail it has to feed.
For instance, a transformerless GT inverter operating purely as "buck" needs DC voltage higher than AC peak. For 120 Vrms AC, that is 170 Vpeak, but voltage can vary and is often higher when being driven.

Your 120V minimum is lower than Vpeak for either 120V or 240V, so I assume some transformer. However, I've seen wide range for other transformerless inverters. There is a "buck/boost" configuration (actually done as "inverting" or polarity reversing, I think)

Best if you select panels adding up to higher Vmp.