Wow, that is a lot of wattage there, Bob. Nice. How many volts is your battery bank? 12/24/48? How many panels specifically and how many volts per panel? Are all the panels the same?
I am going to assume 20 panels at 12 volt and 120 watts per panel. If you wire two in a series, that would give you about 45 volts @ absolute maximum (those rare, perfect conditions when the panel is at peak efficiency). If you go over the safe amount of voltage for even a few seconds, you can smoke the controller(s). An mppt controller that can handle 45 volts from two 12 volt panels wired in series is going to be kind of expensive. At least $300 for a Blue Sky 3024 if memory serves me correct. Even then you could only hook up 4 of those 120 watt panels total for one controller; you would need 5 controllers at $300 a piece to wire in all 20 panels! Ouch!
Morningstar controllers can handle much higher input wattages than Blue Sky or many other high quality brands. The amp ratings are about the same, but the voltage ratings are much higher. One Tristar-mppt-45 can handle two strings of 10, 120 watt panels in series with those two strings wired in parallel and still be well within the limits (that is all 20 of your panels!). The higher input voltage capacity is a big advantage of the Morningstar compared to other brands. The MorningStar Tristar 45 mppt is at least $600 I think, which would be half the cost using several of the comparable controllers from other companies.
If a pwm mated to two 12 volt panels that are wired in series to make 24 volts (real world is about 34 volts), half of the energy will be wasted. The pwm is going to chop the volts down to about 14 so the battery is getting the correct voltage. An mppt will also cut the volts in half, but instead of throwing the excess voltage away, it smooshes that excess voltage into amps. Batteries love amperage especially when they are low. There is a loss when converting volts to amps, but it is much less of a loss than with a pwm. Your 8, 30 amp pwms are well matched to your system if you have 12 volt panels.
When low light conditions limit a 24 volt (actually about 36 volt) panel to produce only 70% of what it can do, it is putting out only 25.2 volts (36 volts X 70%). This is plenty of voltage to charge the batteries; of course fewer amps, but enough volts. If a 12 volt solar panel were only putting out 70% capacity, then it would be putting out only 11.9 volts. (You can have all the amps in the world, but they won't do you any good if you don't have enough voltage to push those amps into your battery bank.) That isn't enough to charge a battery unless it is really low. This wasn't part of your question, but is important to know what happens in less than ideal conditions if you decide to go with wiring two 12 volt panels in series to make, in essence, one 24 volt panel.
MPPT's -with overall watts being equal- are best utilized with 24 volt panels or 12 volt panels wired in series.
PWM's need to be matched to the panel to maximize their limited ability to push voltage into a battery. That means matching 12 volt panels with 12 volt pwm's with a 12 volt battery bank; 24 volt panels with 24 volt pwm's with a 24 volt battery bank. However, 24 volt panels with 12 volt pwm's on a 12 volt battery bank is a massive waste of energy. MPPT is the only way to go when you have 24 volt or higher energy coming in from solar panels to a 12 volt battery bank.
A 12 volt controller means you are putting energy into a 12 volt battery bank. A 24 volt controller means a 24 volt battery bank. It has nothing to do with the solar panels. Yes, total volts from solar panels matters a great deal on a system larger than one 12 volt panel, but that is not how you identify if a controller is 12 volt or 24 volt or 48 volt. You can't wire a 12 volt controller to a 24 volt battery bank.
Buying one controller is cheaper than buying several smaller ones. If you do use several smaller ones, it is nice to have them talking to each other so they act in the best, most efficient way possible. You can have different controllers on the same bank, but they will behave differently and you will lose efficiency. If they operate too differently, that could cause problems. Mixing mppt and pwm is fine, but the settings on the controllers need to be set correctly- I can't remember exactly how that works, though. I match my stuff- more efficient.
Blue Sky controllers with the IPN ability will talk with one another even if the controllers are different models. This makes them function as one controller. Several controllers all on the same page are better than several controllers doing slightly different things. If they function as one controller, why not get one controller as it is cheaper and has less wiring? You lose redundancy. If one controller goes down, you are completely out of the game. Like the setup you have now, at least you are getting some juice. Some of the Morningstars also talk to each other.
I believe Blue Sky can be adjusted without a computer. The Morningstars that can be adjusted need to be connected to a laptop with the appropriately downloaded software. It is very easy to do and use, but it needs a computer. The computer needn't be hooked up all the time; it's to set the original parameters.
If you are getting enough electricity, I would live with the system and get me another cheap pwm. If you are wanting to increase the amount of electricity you are getting, you could benefit massively from mppt with the large amount of solar you have.
I'm too new at this to be super confident about my answer. You know a lot of this stuff, but many readers won't, so that's why I went with more detail and explanation. I also don't have specific numbers, so I can only estimate. Your question is a good one that a lot of people will want to know about. Am looking forward to learning a thing or two.
