VOC, VMP, LSC, and matching your array to your inverter...

[Brooklyndodger]

I'm hoping someone can confirm or correct my understanding of how to match your solar array strings to your charger/inverter. This is what I think is true:

Use VOC to make sure you do not exceed your inverter's capacity. Panel VOC x number of panels in your string x 1.2 (a rough constant to adjust for cold weather voltage boost) should be less than your inverter's max DC input voltage rating.

Use VMP to make sure you meet your inverter's MPP startup threshold. VMP at highest rated ambient temperature (since voltage drops as temperature increases) x number of panels in your string must be equal or greater to your inverter's lowest MPP operating voltage range.

Use LSC to ensure you do not exceed your inverter's maximum current capacity, that is, the maximum amps your controller can take. So LSC x number of parallel strings (if you are just running one string of panels in series the lSC is as rated for a single panel) must be lower than your inverter's maximum short circuit input current.

Is this roughly correct? Setting aside sizing an array according to your energy needs etc., etc., are these three factors what you need to use to make sure your inverter has enough juice to charge your batteries while not going boom?

Thanks in advance!

P.S. Is there a rule of thumb for determining lowest likely VMP of a string based on varying ambient conditions like there is one for estimating the highest VOC?

 

[sunshine_eggo]

Use VOC to make sure you do not exceed your inverter's capacity. Panel VOC x number of panels in your string x 1.2 (a rough constant to adjust for cold weather voltage boost) should be less than your inverter's max DC input voltage rating.

This works. It can be refined by using your climate's record low temperature and your panel's Voc temperature coefficient.

Use VMP to make sure you meet your inverter's MPP startup threshold. VMP at highest rated ambient temperature (since voltage drops as temperature increases) x number of panels in your string must be equal or greater to your inverter's lowest MPP operating voltage range.

High temps will cause Vmp to be lower, so maybe apply 0.8 * Vmp to insure you stay above MPPT minimum.

Use LSC to ensure you do not exceed your inverter's maximum current capacity, that is, the maximum amps your controller can take. So LSC x number of parallel strings (if you are just running one string of panels in series the lSC is as rated for a single panel) must be lower than your inverter's maximum short circuit input current.

Yes.

Supplement by also comparing Imp to the maximum usable PV input current. Many MPPT list a maximum usable current and a maximum short circuit current.

P.S. Is there a rule of thumb for determining lowest likely VMP of a string based on varying ambient conditions like there is one for estimating the highest VOC?

It's the same coefficient. In the absence of actual panel data, -0.4%/°C is conservative. For every 1°C above/below 25°C, the voltage will drop/rise 0.4%.

 

[zanydroid]

Use VOC to make sure you do not exceed your inverter's capacity. Panel VOC x number of panels in your string x 1.2 (a rough constant to adjust for cold weather voltage boost) should be less than your inverter's max DC input voltage rating.

Use VMP to make sure you meet your inverter's MPP startup threshold. VMP at highest rated ambient temperature (since voltage drops as temperature increases) x number of panels in your string must be equal or greater to your inverter's lowest MPP operating voltage range.

Looks good overall. (It's Isc btw not Lsc -- I as in current, short circuit)

That said I think VOC is what you compare to startup and VMPP should be compared to operating range.

If either VOC or VMPP is below the respective specs your MPPT will stop converting.

I don't know how common it is for VOC headroom to be respected but VMPP above max operating voltage, or what happens at that point.

 

[chrisski]

Use VMP to make sure you meet your inverter's MPP startup threshold. VMP at highest rated ambient temperature (since voltage drops as temperature increases) x number of panels in your string must be equal or greater to your inverter's lowest MPP operating voltage range

Kind of. Also, a cloudy day may make the voltage not as high as normal. I’d give it some play for that. I haven’t collected enough data in Sunny AZ to say for sure.

Was overcast nearly the whole day today and my panels produced the rated wattage, and for the last few days with no clouds they had put out three times there rating.

 

[Brooklyndodger]

Yes.

Supplement by also comparing Imp to the maximum usable PV input current. Many MPPT list a maximum usable current and a maximum short circuit current.

Thanks for your response and this addition. This addition seems pretty important since, say, my panel's IMP is 11 amps and my inverter has a max usable input current of 17 amps and a max short circuit input current of 25 amps (like the eg4 6000xp), even though I could theoretically parallel the panels without exceeding the short circuit input rating I would be exceeding the maximum usable current. All of these acronyms are starting to make sense...

 

[zanydroid]

Thanks for your response and this addition. This addition seems pretty important since, say, my panel's IMP is 11 amps and my inverter has a max usable input current of 17 amps and a max short circuit input current of 25 amps (like the eg4 6000xp), even though I could theoretically parallel the panels without exceeding the short circuit input rating I would be exceeding the maximum usable current. All of these acronyms are starting to make sense...

It could still be worth paralleling, there are a lot of worthwhile scenarios.

IMP @ STC is an optimistic number

For instance you could parallel together strings with different orientation, and they will peak at different times.

If you want to increase winter / shoulder season solar harvesting, paralleling will achieve that, despite clipping in the summer.

If you have a hard kW-AC limit imposed by a grid tie authority overpaneling behind the limit can be the optimal strategy

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