Wattage vs Voltage vs Current on Inverter Ratings

[ebrummer]

Theres so many great resources here and I've watched a lot of videos. Forgive me if this is covered somewhere and I missed it.
Electrical calcs are typically Watts = Volts * Current.

When sizing out a system, if you look at the specs on a lot of off-grid inverters, there will be a max Voltage, a max current and a max wattage. In strict math terms without factoring reality, one of those numbers is over constraining the problem.
So, if I'm sizing out a system, and lets use the EG4 6000XP system for example:
Max PV Input: 8000W (4000W across 2x MPPTs) although elsewhere on the spec sheet it recommends 10000W max...
Max voltage: 480V,
Max usable current: 17A,
Max short circuit current: 25A

So, if I size my system by using the Voc of the panel and the Isc to meet the current and voltage requirements, that's often over the listed Wattage requirement.
For example imagining we have a single 480V panel at 17A you could theoretically get 8160W on a single MPPT which is double the rated wattage. This is a very simplified example, but I often find, if I size based on the Voc and then depending on the panel current, do 2 strings in series so lets say 4S2P or 6S2P etc, I can meet the current and voltage requirements, but will be over slightly on the total wattage. (And thats without the spec sheet listing 8kW as the spec and also mentioning 10kW.) A lot of the information I've read on planning systems doesn't talk as much about the total wattage limit. Then you factor in that using Voc as long as you're not in the cold, is higher then I'll ever see, the total wattage of the panels will be much lower then calculating wattage with Voc and Isc. Should I be using Max power voltage and max power current when comparing wattage of system to wattage of inverter PV input? I'm trying to max out my PV wattage input based on the inverter and panels I'm looking at. Then you have over paneling, where once you exceed a certain current, you're not adding usable current to the system (so long as the system won't fail with that extra current.)

 

[wattmatters]

Should I be using Max power voltage and max power current when comparing wattage of system to wattage of inverter PV input?

Max voltage (at the coldest temperature expected for your area) is the one limit you must not exceed. Best to give yourself some safety margin there.

That said, don't exceed the max short circuit current rating either.

Wattage is a bit like the area of a rectangle while current and voltage are the lengths of the sides. Each side has a maximum length, while you are also trying to maximise the area of the resulting rectangle.

Panels of the same wattage rating come with different voltage and current ratings. Some have lower voltage with higher current while others have higher voltage with lower current. It helps to use panels and string designs which are a suitable match for the MPPT input specs of the inverter/charge controller.

e.g. you might go for panels with a Isc of no more than 12A in a 2P set up but if you had panels with an Isc of 15 A then you would be restricted to a 1P string.

 

[MichaelK]

Don't waste this much time overthinking everything. You really only need to concern yourself with two details. Overpanel the amperage, and a few amps gets occasionally wasted. Exceed the Voc, and your electronics short-out, catch on fire, and maybe people die in the fire.

 

[Hedges]

Most of the time panels put out less than spec wattage, and panels are cheap now, so it is desirable to over-panel.
It isn't unreasonable to put 9000W of panels on a 6000W inverter, likely will rarely reach 6000W output without drawing from battery. 12,000W of panels oriented one direction would be reasonable. 16,000W of panels, with strings oriented 2 directions, would supply full power the inverter could handle more hours.

Given 2x MPPT of 4000W each, consider 8000W of panels on each MPPT, with 4000W facing SE and 4000W facing SW. Total of 16,000W and each MPPT has multiple orientations.

If more than 2 strings of panels per MPPT, use fuses or breakers per string.

https://eg4electronics.com/wp-content/uploads/2024/04/EG4-6000XP-Inverter-Spec-Sheet.pdf

"DC INPUT VOLTAGE RANGE 100-480 VDC"

"NOMINAL MPPT VOLTAGE 320 VDC"

Never exceed 480V from PV, even in freezing weather. PV panel Voc x number of panels in series should not exceed 414V, so if it rises 16% in cold weather it remains below 480V (there is a calculation involving temperature coefficient of panels to cut that closer if desired.)

Aim for 320Vmp in hot weather to maximize efficiency of MPPT, avoiding it running too hot so it lasts longer. PV string totaling 370Vmp (STC standard temperature) might be about 320V when hot, assuming similar 16% change and no shading on any panels.

 

[Mattb4]

...

So, if I size my system by using the Voc of the panel and the Isc to meet the current and voltage requirements, ...

Use Imp. Isc only exists as a panel dead short condition. The max current carrying rating of a SCC I would assume are based on the ability of the SCC to internally operate at that level for a reasonable time period.

 

[wattmatters]

It isn't unreasonable to put 9000W of panels on a 6000W inverter

Personally I would stay within the inverter's MPPT power input rating. If that rating is inadequate then get an inverter with a higher input rating.

Given 2x MPPT of 4000W each, consider 8000W of panels on each MPPT, with 4000W facing SE and 4000W facing SW. Total of 16,000W and each MPPT has multiple orientations.

But if I were to do it, this is the way I would do it. Probably with a panel tilt to favour Winter over Summer. Assuming one has a choice over panel azimuth and tilt, which they may not if it's just going on the roof.

 

[ebrummer]

Most of the time panels put out less than spec wattage, and panels are cheap now, so it is desirable to over-panel.
It isn't unreasonable to put 9000W of panels on a 6000W inverter, likely will rarely reach 6000W output without drawing from battery. 12,000W of panels oriented one direction would be reasonable. 16,000W of panels, with strings oriented 2 directions, would supply full power the inverter could handle more hours.

Given 2x MPPT of 4000W each, consider 8000W of panels on each MPPT, with 4000W facing SE and 4000W facing SW. Total of 16,000W and each MPPT has multiple orientations.

If more than 2 strings of panels per MPPT, use fuses or breakers per string.

Thanks, and thanks to everyone else.
I should clarify that, I'm specifically working on a Burning Man array that'll be transported, constructed and torn down, although Ill use the panels/inverter setup in other ways elsewhere.
So my low temperature is warm, so I have no risk of really cold temps overvolting the system. And the coldest temps will be at night not day when trying to generate power. I am limited in size capacity and $, transporting and setting up panels and buying panels. I currently have 28x 320w panels. (64.8 Voc, 54.7 Vmpp, 5.86A) combined with a 6000W inverter w/ 8000w PV input (EG4 6000XP: 2x MPPT, 480V max per mppt, 17A max, 320v "ideal")
So using 2x 7s2p: I get 454 Voc max in series and 12.48A which meets the input for each MPPT but is technically 5661W (Voc * Current) which exceeds the 4000W claimed max watts per mppt.

As you all have said, panels won't actually hit that perfect Wattage due to temp/sun/dust etc. So it sounds like I'm just barely over-paneling with this setup and it'll work fine? And technically I could do even more if I wanted to try to get closer to the actual max inverter spec during normal use conditions of the inverter/panel array? I've seen some people throw out guidelines for overpaneling and others basically say as much as you want.

 

[Hedges]

Temporary - maybe you'll lay them flat on the ground. But drive stakes so they don't blow if wind kicks up.

Yeah, overpanel as much as you feel like. Just don't go over voltage. Whatever coldest temperatures at night, panels could retain that temperature when light first hits them in the morning.

 

[ebrummer]

Temporary - maybe you'll lay them flat on the ground. But drive stakes so they don't blow if wind kicks up.

Yeah, overpanel as much as you feel like. Just don't go over voltage. Whatever coldest temperatures at night, panels could retain that temperature when light first hits them in the morning.

Thanks. Yea they'll be either mounted on shade structure, trailer or a frame or on the ground and secured down. High winds and tents taking to the sky aren't uncommon for people who don't pay attention. 18" lag screws driven into the ground work well for securing things. I got a good deal on those panels but they're old sunpower panels with fiberglass frames that are not great for handling unfortunately.

 

[Hedges]

Some SunPower panels degrade rapidly due to PID if biased positive relative to earth.
Might not matter during Burning Man (especially if no rain), but for future use consider positive ground inverters like SB 5000US series.
If these panels are susceptible. I think newer ones that are resistant would say "PID Free" on data sheet.

 

[ebrummer]

I saw some stuff about really old sunpower panels. Mine are old but not super old. Although no PID mention on the data sheet. SunPower-SPR-320E-WHT-D. They're putting out good voltage when testing. Waiting for a few pieces to come in to test power generation. Hopefully they're OK. Thanks again for everything.

 

[pollenface]

That said, don't exceed the max short circuit current rating either.

can you fuse to the max short circuit rating to mitigate this? I find myself asking myself...

 

[wattmatters]

can you fuse to the max short circuit rating to mitigate this? I find myself asking myself...

You could. Just easier to stay within limits and eliminate the chance of failure to begin with.

 

[Hedges]

I saw some stuff about really old sunpower panels. Mine are old but not super old. Although no PID mention on the data sheet. SunPower-SPR-320E-WHT-D. They're putting out good voltage when testing. Waiting for a few pieces to come in to test power generation. Hopefully they're OK. Thanks again for everything.

My 327W E20 series claim PID free. I would have thought 320W were similar generation, but ...

https://149795217.v2.pressablecdn.com/wp-content/uploads/SunPower-SPR-320E-WHT-D.pdf

Sunpower positively grounded modules

Was wondering if anybody can help me understand the sunpower positively grounded modules? I am assuming it is a way they manufactured the solar cell. I understand also with the older transformer style inverters the inverter manufacturer needed to make an adjustment internally to be compatible...

forums.mikeholt.com

Then again, this one:

http://eshop.terms.eu/_data/s_3386/files/1365425216-SUNPOWER_SPR_320NE_WHT.pdf

says, "TRANSFORMERLESS INVERTER COMPATIBILITY Comprehensive inverter compatibility ensures that customers can pair the highest efficiency panels with the highest-efficiency inverters, maximizing system output"

So probably OK.

label each of your panels 1 ... n or some way to track the order in which they are series connected. That way you can see later if the more positive ones have degraded faster.

There were some schemes to reverse part of the effect by biasing the panels to negative voltage during the night. Reconnecting wires so panels are in the opposite order periodically might do the same when used with transformerless inverter. Figuring out if there is a problem would involve PV panel tester.

 

[pollenface]

You could. Just easier to stay within limits and eliminate the chance of failure to begin with.

Hobbiests don't always do what's easiest though...

 

[ossiemoore]

Don't waste this much time overthinking everything. You really only need to concern yourself with two details. Overpanel the amperage, and a few amps gets occasionally wasted. Exceed the Voc, and your electronics short-out, catch on fire, and maybe people die in the fire.

Love you you described this. You made me chuckle.