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.