pvdude,
Have you done a reasonable energy audit of how much energy you really use while the sun is up and while the sun is down?
My system is using a single XW-Pro battery inverter, but all of my solar is currently AC coupled from Enphase microinverters. I am also grid tied, but I have been doing the math to figure out what I would need to do to eliminate using grid power unless something goes wrong.
The XW-Pro is a beast of an inverter, and it will support cranking out 6,800 watts all day long. That is a true continuous rating. The surge rating is 12,000 watts. To hit that urge, your battery bank will need to supply about 250 amps into the inverter. At 6,800 watts, it is still pulling about 140 amps. In my case, a single XW-Pro could run everything in my house with only one exception, my central air conditioner compressor has a 105 amp starting surge. So I left that on the main grid panel. With a soft start unit, it might be able to run off the inverter, but for the few times my grid has gone down, I can deal without having air conditioning.
If you truly need more than the 6,800 watts continuous, then two or even 3 XW-Pro inverters may be needed, but that is a huge amount of power.
The next thing to look at is the total watt hours used in a day. And then splitting it up to what is used when the sun is up, and when the sun is down. With all DC coupled solar, all of your AC power is going in and back out of the batteries, all of the time. The battery is a buffer. In reality, yes, the inverter is using some of the solar that does not make it into the battery, but because of how the inverter works to make a true since wave, it is pulling the power in cycles making the top half, and then the bottom half of the sine wave. At zero crossing, the solar is charging the battery, at peak, the inverter is pulling a bit from the battery. At 60hz, there are 120 current pulses per second.
In my case I am completely AC coupled. When the sun is up, my Enphase microinverters are powering all the loads directly. I have the charge rate set so that the XW-Pro is only using power that I am not using in my home. This power would have been exported out to the grid. I have it charge at just 25 amps or so, over 8 hours while the sun is shining. I know, that is only 200 amp hours. I do want to step this up a bit, but I need to add more solar. I only have 4,800 watts of panels, and 3,900 watts of solar grid tie inverter output. I am 25% over paneled to the inverters. They only hit clip on very sunny but cool days. I produce up to 31 KWHs a day in ideal conditions, my average is about 23 KWHs a day. When the sun goes down, I run my home off of the battery bank, and it supplies most of my needs. But I do fall short when I am running the Central air conditioning.
After much thinking and research, I have come to the conclusion that having both DC and AC coupling makes the most sense.
My existing microinverters supply more than the power I need while the sun is up. but it is a pain to regulate how much to push into the batteries. And the fact that the XW-Pro software has no way of switching into charge mode on it's own. I really feel this is a bad software issue, but that is another thread. So my plan is to add a DC solar array and charge controller that can supply the power I use at night, when the sun is down. I want to add 2,000 to 4,000 watts of DC solar panels. The XW will basically sit in standby from sunrise to sunset. The DC charge controller will harvest all it can and charge the battery bank. If the batteries do become full, it will throw away any additional production, nothing I can do about that, except maybe a load dump like an electric water heater. As the sun falls, the XW-Pro will come online and power my home off the stored power in the batteries from the DC solar.
It really becomes 2 separate systems. AC solar provide all the power during the day, and the DC solar provides all of the power at night. If you do need some crazy power at say 2 pm, to run a welder, then the XW-Pro could go into "Grid Support, and add it's up to 6,800 watts to the 3,000 watts that are coming from the Enphase microinverters. And the DC solar, will be providing some of the power, so the batteries will not be discharging very fast. When the sun is down, you are back to being limited to just the XW-Pro output. But in my case, being "Grid Tied", If I need more power than my inverters, it will just pull from the grid, and if I am producing more than I need, it can sell out to the grid. As cool as it would be to go fully off grid, having the grid there is like having a massive extra battery bank available when things are not balanced. Off grid, you really need to balance your production and load much closer.
As I have been thinking about this for nearly a year now, since I put my XW-Pro into service, I truly believe the combination of AC and DC coupling is the best way to go. For both on and off grid systems. Direct Solar to AC inverters, either string or microinverters are still the most efficient way to get AC power while the sun is shining. But they do not store extra power very well. But as I learned with my latest grid power failure, the XW-Pro does work great when off grid, and took all of my extra solar and was able to push that into the batteries, but at about 85% efficiency. That is way better than just throwing it away though. But having to convert DC from the solar to AC, then back to DC to charge the batteries, and then back to AC to run loads... That is a lot of conversions. Not take DC solar.... When the sun is up, you are taking the solar array high voltage, stepping it down, and regulating it to charge the battery bank, and the inverter has to work off this low battery voltage and step it back up and form an AC sine wave. That is not the best efficiency either. But when you can time shift the DC power from when the sun is shining to when the sun is down, then it becomes the best way to get the power at night. Due to roof space limits, I may actually take a few of my AC panels, and remove the microinverters and use them as DC panels. I typically use a little more power when the sun is down compared to when it is up. I have 4,800 watts of AC panels now, but only have room for about 3,000 watts more. Since I want more power at night, I might benefit from only 3,600 watts of AC panel, and 4,200 watts of DC panel. but this is my use case, yours may vary. Do some energy use calculations and see when you are using the power, and when the solar is producing. I firmly believe, that combining the right balance of AC and DC solar will produce the best system. And this is true for both on and off grid.
As for the original post here, you certainly can add more DC charge controllers and solar panels, but once you hit the maximum charge rate of the batteries, you have a problem.