The Victron will still be the heart of your system (so to speak) giving frequency when the grid drops etc. The Fronius isn’t connected to any batteries. It is just a PV inverter, making AC from the panels connected to it. In a Victron system, you could use it instead of an MPPT. If you watch the ESS video on Victron’s youtube channel, I’m pretty sure they explain how they integrate with Fronius.
Think of it like this:
- The Victron inverter is you main source of power. It is connected to the batteries and all your critical loads are on its backup output.
- One or many MPPTs sits directly on your DC bus, effectively charging your batteries. They do not add to your total inverting capacity, only play the role of charging the batteries. The Victron inverter takes power from the DC bus if it needs to invert.
- You and put a Fronius (or some other brands) PV inverter on the AC input of the Victron, or on its backup AC output and connect some panels to it. If the Victron wants to charge its batteries, it can get some power from the Fronius. The Fronius can also serve to power loads. The Victron communicates directly with Fronius to control it.
@Sarel.Wagner has a system like that, if I’m not mistaken? Here’s a link to Sarel’s system if you want to have a look: My Micro grid system
Effectively, think about it like this:
- Using power in the daytime (not charging batteries) is 90% efficient if you do it with the Fronius. Charging batteries is about 80% efficient with the Fronius. But then you also need to discharge them to use them, dropping the full cycle’s efficiency to 70%.
- Using power in the daytime or charging and subsequently discharging batteries with MPPTs are 80% efficient. But critically, MPPTs doesn’t add additional inverting capacity.
There are obvious limitations of relying on a PV inverter to supplement your inverting capacity, but should you investigate going that route, get a good installer to deal with it, like @JacoDeJongh