One or two inverters?

I have a system with 42 LG400 panels that can output about 15.2kW. The highest Solar Edge inverter seems to output about 11.4kW. Should I put in two Solar Edge 7.6kW inverters?

Is that what's commonly done or is it best to search for a bigger inverter?


Edit: If two inverters is the common way to go, what's the best string configuration...3 strings per inverter with 7 panels per string?

thanks
 

electrofelon

Senior Member
Typically your inverter capacity is less than array nameplate. This is commonly called the DC/AC ratio. It's a bit of a science and an art to find the right ratio, but 1.2 is a ballpark for my climate and typical parameters. You'll have to find an inverter or combination that gives you close to that target number. Cost, brand preference, and availability come into play too.
 

ggunn

PE (Electrical), NABCEP certified
Typically your inverter capacity is less than array nameplate. This is commonly called the DC/AC ratio. It's a bit of a science and an art to find the right ratio, but 1.2 is a ballpark for my climate and typical parameters. You'll have to find an inverter or combination that gives you close to that target number. Cost, brand preference, and availability come into play too.
Some jurisdictions put a cap on DC:AC, as do some inverter manufacturers, but I have seen systems with ratios up near 1.5 depending on location and orientation. For a system way up north pointed significantly away from due south the modules will never see anything near STC.
 
Thank you for your help.

Is it fair to say that for locales with less sunlight the ratio can be higher say 1.2-1.5 and for locales with more sunlight (arizona) you'd want the ratio closer to 1?

Lastly, If I want to use double inverters and I have 42 panels. Can I have 2 strings per inverter with one string w/ 11 and the other with 12? Anything wrong with this or do the strings need to be equally balanced?
 

Carultch

Senior Member
Do an hourly PV Watts, and determine what the maximum power the array is estimated to produce throughout the entire year. Use the max() function in Excel, but make sure you exclude the sum total annual value at the bottom of the spreadsheet. Add more inverter capacity to mitigate the risk of clipping, if this exceeds to inverter kW rating.

You need uniform string sizes within any given MPPT power processing zone. Unless otherwise specified, inverters in general only one zone for this. Some have multiple zones, allowing more design flexibility. For Solaredge inverters specifically, the inverter design externalizes this functionality to the DC optimizers, that are installed at the module level, and therefore allow string sizes to differ on a given inverter, provided the minimum size is met, and no strings exceed the maximum DC Watts per string.
 

jaggedben

Senior Member
Use Solaredge's new online software. It states clearly what the 'clipping' will be due to the DC/AC ratio. In some situations, for example half of the system facing East and other half West, you wouldn't get any clipping with the smaller inverter. Decide based on customer expectations how much clipping you can live with.

7 panels in a string is below the minimum size for solaredge. Also the strings don't have to be the same length.
 

electrofelon

Senior Member
Some jurisdictions put a cap on DC:AC, as do some inverter manufacturers, but I have seen systems with ratios up near 1.5 depending on location and orientation.
True. We have discussed this in a past thread, but there is potential problem with very high DC/AC ratios where the inverter runs out of "real estate" on the IV curve to clip the power needed and has to shut off completely.

For a system way up north pointed significantly away from due south the modules will never see anything near STC.
Thank you for your help.

Is it fair to say that for locales with less sunlight the ratio can be higher say 1.2-1.5 and for locales with more sunlight (arizona) you'd want the ratio closer to 1?
I dont think you can say that. In fact I would say its the opposite and further north is where one might see the most clipping due to colder temperatures. Its also no about whether you get clipping or not, its about the duration and intensity of clipping and whether those losses will be made up in less inverter capital cost. I am at 43 degrees north with a south facing relatively steep array (36 degrees). Open racking which has more airflow. 1.15 DC/AC ratio. I see my inverters at max very frequently. Its a little disconcerting, but the software (pvsyst, they have a free trial period) said the losses are minimal, and the $200 upcharge for the next size inverter would have a very long payback.

Lastly, If I want to use double inverters and I have 42 panels. Can I have 2 strings per inverter with one string w/ 11 and the other with 12? Anything wrong with this or do the strings need to be equally balanced?
Your inverter manufacturer will have a "string sizing" calculator that will tell you the allowable configurations. It mainly based on temperatures which determine voltage. Too low and on hot summer days you can drop out of the inverter's capability. Too high and you exceed to voltage limits of the conductors and equipment. As JB said, different MPPT inputs can receive different voltages. I have two strings of 12 and one string of 11 on mine.
 

ggunn

PE (Electrical), NABCEP certified
True. We have discussed this in a past thread, but there is potential problem with very high DC/AC ratios where the inverter runs out of "real estate" on the IV curve to clip the power needed and has to shut off completely.
Of course, but if the modules never see more than 500W/m^2, at DC:AC = 1.5 the inverter won't run out of capacity to adjust the iV curve.
 

electrofelon

Senior Member
Of course, but if the modules never see more than 500W/m^2, at DC:AC = 1.5 the inverter won't run out of capacity to adjust the iV curve.
Yes, you did say "pointing significantly away from South" in your previous post. That could certainly make for a high dc/AC ratio with no or little clipping. A low angled northerly array might be somewhat "self regulating" with the low temps coinciding with the lower sun and little albedo. My array being steep gets close to orthogonal irradiance during the cold months, and a good amount of albedo.
 

wwstrick

Member
You could use a Solaredge 11.4kW HD Wave. That would give you a 1.33 dc/ac ratio. That would work fine for me here in Guatemala, but you need to consider your own location. Your string size is determined by Watts per string and can be found on the optimizer spec sheet.
 
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