Optimizing Panels for EG4 3kW Inverter

[boarder2k7]

I am looking to set up a small system using a pair of server rack batteries and an EG4 3kW inverter to power some critical loads and act as a battery backup for them.

The plan is to have this be double conversion not directly tied to the AC to avoid any issues with my power company, so I'll be also using the EG4 Chargeverter to keep the batteries topped as needed. The idea is to set the Chargeverter to keep the batteries charged to approximately 50% so I always have backup power available, and let the inverter charge the batteries to 100% off solar as it is available.

I'm looking at panels, and I have a local source for 445 watt Ureco panels, which are a 42V panel, 35V at max power. Looking at the spec sheet for the 3000EHV-48 it says that MPPT is from 120-450Vdc. Am I better off going with smaller wattage panels to up the string voltage to better fit the upper end of that MPPT range? I'm only looking at adding about 2 kW of panels in total for this project. Thinking that more lower wattage panels will keep the string voltage up when the sunlight is less than direct, thus allowing better MPPT utilization.

Edit: Alternatively I have another local source for really cheap 325 watt panels with the caveat that they're 9 years old. Is there any downside to just stacking up with $0.13/W used panels?

 

[MaikaiLifeDIY]

Either method works so long as the voltage is descent enough to push the current into the MPPT. I typically try to hit about 80% of the max voltage for the MPPT, it's just a safe number that I've settled on, but can go lower or higher if needed.

I would also suggest looking at optimizers if you want the most energy out of your panels, in a string inverter configuration your higher output panels (the ones getting more sun) will be limited by any lower output panels (the ones not getting the sun) which overall brings down the output of your strings.

 

[zanydroid]

Test the used solar panels first and see if they are good.

Used = looks worse, worse use of available area because of lower efficiency, more racking cost. Other than that, solar is solar.

Consider also overpaneling by adding panels in parallel; this will also hedge against shading.

AFAIK (and I’m willing to be corrected) Higher string voltage mainly helps with horizon shading causing more bypass diodes to activate. As diodes activate panels drop below the start limit. Also helps with high temp voltage sag.

Not sure why smaller wattage would mean higher voltage, it all depends on how many cells in series there are. Sure if you buy older panels with lower wattage per cell the voltage will be higher for the same module level wattage.

 

[boarder2k7]

Thanks @MaikaiLifeDIY optimizers are a great thought to include. I'll have to look and see if I can get them with rapid shutdown built in to the optimizer as well.

@zanydroid Is there any good way to test other than just for voltage? I guess I could try a short circuit amperage as well?

More panels is great but I'm limited by space right now, trying to mount these unobtrusively.

I was thinking (possibly incorrectly now that I've looked a bit further) that the panel voltages would be similar between different wattage panels so smaller panels would allow me to fill the same physical space and get higher overall voltage.

I'm waiting on some information from the seller with the new panels. I noticed that they are not UL stamped so I'm not sure if I'd have a problem if inspection was needed.

 

[zanydroid]

@zanydroid Is there any good way to test other than just for voltage? I guess I could try a short circuit amperage as well?

I haven’t bought many used panels. You should be able to find solar panel testers, and reviews of those.

I'm waiting on some information from the seller with the new panels. I noticed that they are not UL stamped so I'm not sure if I'd have a problem if inspection was needed.

Surprised there would be that many non UL stamped panels from 9 years ago. What about other NRTL stamp? Post the spec label , it should be on there.

I was thinking (possibly incorrectly now that I've looked a bit further) that the panel voltages would be similar between different wattage panels so smaller panels would allow me to fill the same physical space and get higher overall voltage.

Voltage is primarily determined by number of cells in series. Which is somewhat related to wattage but I don’t think that is the way to get the intuition.

Older panels will likely have similar range of voltage as newer ones but physically smaller and less efficient modules. If you are space constrained you may not want to go down from today’s 21% efficiency to whatever efficiency was 9 years ago (17%? That would require 25% more area to compensate)