Ok, so that put my mind in a bat slide. So I’ve got 15kW on the primary MPPT’s and 5kW on the AC-coupled side, times two inverters for a total of 40kW. So in a grid outage situation, I will only have 24kW of solar going to the batteries/consumption? Hmmm? I think I’ll still be ok, but definitely something to watch.
Note:
The Minimum Battery Power limit for this configuration should be 2 * Max PV output. This will allow excess AC coupled power to be managed without causing voltage spikes in the DC system. So your batteries should allow for 80KW. So if you are using batteries that allow 100 amps you should have 16 of them. The rest of your DC design should allow for 800 amps. So probably parallel 4/0 cables, 1 for each inverter. This would meet the cabinet rating for 4/0. Electrical code in USA would actually require 500 mil. And at least an 800 amp rated DC bus.
When we deal with electricity we deal with "Power" and "Power Limits". Power is the rate at which energy is produced. That will be either watts or KW (kilowatts). Energy is Power over time. If I use 1 KW for 1 hour I have used 1 KWH. So these system have limits on Power which is the maximum rate a which they can use energy. Batteries have limits on both power and energy. Maybe the max load on a battery is 100 amps or 5 KW. If the battery can support this load for 1 hour, then it can provide 5KWH of energy.
First you need to understand this. In DC coupled solar, the power first flows into the DC bus / Batteries. These 18K units allow up to 18KW each to flow into the batteries through the MPPT chargers. The MPPT charger is one component of the All-in-one unit that we often refer to as an Inverter. In your case the MPPT chargers allow up to 30KW of DC to be harvested. I don't know the exact figures for your area, but that should allow you to capture up to 30 * 5 = 150 KWH on a typical sunny summer day. Your 10 KW AC coupled would add another 50 KWH for a total of 200 KWH. These numbers are probable conservative. That is how much "Energy" your system will produce in a day. If you are on grid this excess energy can be exported. The 10 KW of AC coupled will supply you house first and then flow to the grid if not used.
Now before you can Export or Use energy from the DC system, it will need to be converted into AC. It is OK and in fact desirable that you can harvest power faster than you can use that power. Your system will only harvest power for 10-12 hours a day. Also your peak power production is only about 2-3 hours a day. The system will need to supply power to the loads for 24 hours a day. When on grid this is not problem, you export the excess and then draw it back again during the night. You can also do Self Consumption where you charge your batteries during the day, and then use that energy at night.
So this pretty much covers the way these system capture energy. So now lets look at the consumption side.
There is a component piece of hardware in your unit called an "Inverter / Charger". We have gotten used to referring to any All-In-One unit as an "Inverter", but really these devices have a bunch of hardware devices bundled into them. So when I say "Inverter / Charger" here I am not talking about the entire box, but only the portion that turns DC power into AC power or AC into DC. This Inverter / Charger is what converts the DC in the batteries or coming in from the MTTP chargers into useable AC power. With two Inverter / Chargers you will have 24KW plus what ever you get from your AC coupled power available to use. 24KW is a LOT of power. In my house my usage is typically less then 1 KW unless there is a major appliance of some sort turned on. Even my AC only draws around 2KW. This 24KW limit is going to be an issue only if you are off grid and trying to use battery power or if you are on grid and pulling power out of the batteries to export it. Since your system can only produce about 150 KWH in a day, you could export all that power to the grid in just 6 hours. When on grid you can still pull as much power as you need from the grid.
So lets say we are off grid. Our load is drawing 15 KW. Our AC coupled is producing 7 KW. The inverters will draw 8KW of power from the batteries to produce the gap between the loads and the 7 KW the AC coupled is producing. As long as your batteries are charged, the 24 KW will always be available from the inverters. The AC coupled power will vary throughout the day and depend on cloud cover. Now lets say our big AC units shut down and our load drops to 3 KW. The AC coupled is still producing 7KW. Now the Inverter / Charger switches to its charger mode. It will push 4 KW into the batteries. These devices either draw power from the battery or push power into it. They can't do both at the same time. The inverter can manage the output of the AC coupled array through frequency shifting, but this is a slower and less precise process than the one used by the MPPT chargers.
So when you are off grid and the batteries are full, the Unit may disconnect the AC coupled power since it is harder to manage and not needed to keep the batteries full. That is why they put this on the Gen input. They can use the relay to shed the excess power when there is no more room in the batteries. So the fact is when off grid you can only count on having the 24KW that is available from the inverter / charger. The unit will continue to harvest solar on the MTTP chargers at whatever rate it required to keep the batteries topped off. That will mean you have to manage large loads like air conditioners, electric driers, and water heaters so they do not exceed 24KW. If you think you will spend a lot of time off grid, then you really should be looking at gas or propane appliances. A gas water heater and drier costs a little more than an electric. A gas stove about the same. The inverters and batteries required to power these units off grid costs thousands. Load reduction is much cheaper than buying bigger inverters. Another thing you will want is a soft starter for any large motors such as air conditioner compressors. The starting surge for these motors is often 5-10 times the running amps. So if you have a large AC with 10 running amps. It can spike to 100 amps when starting. That would use up all the available power from your inverters. These soft starters might lower that number to 30-40 amps.
Generally, your system will work better off grid when your AC coupled < DC coupled. In your case the DC is much larger than the AC coupled, so you are in good shape there. Your total power output from solar is 40KW. 30KW goes through the MTTP chargers and 10KW goes through the Inverter/Charger device. The Total AC coupled must be less than the Inverter / Charger rating, so 10K < 24K. That should be fine.
The Minimum Battery Power limit for this configuration should be 2 * Max PV output. This will allow excess AC coupled power to be managed without causing voltage spikes in the DC system. So your batteries should allow for 80KW. So if you are using batteries that allow 100 amps you should have 16 of them. Your DC design should allow for 800 amps. In other words the batteries need a reserve that will not actually be used. So at least parallel 4/0 cables, 1 for each inverter. This would meet the cabinet rating for 4/0. Electrical code in USA would actually require 500 mil. And at least an 800 amp rated DC bus.
