Here's my 2p worth, although
@rpdom has covered most items already....
I'm thinking of adding a second inverter to my setup. I have a 4.6kW Solis RHI 5G with 9.6kW battery, I'll be adding ~1.5kW additional inverter (no batteries). There are threads elsewhere that suggest this is possible
Yes, that configuration is possible - I have similar configuration working with two Solis inverters.
I want any excess solar to charge the batteries, whichever inverter is generating it. How do I do that?
You don't need to do anything specific.
Let's assume your house load is using 500W, your additional inverter is generating 1000W and your RHI is generating 2000W, but the batteries are only half full.
The RHI will detect export to grid via your grid power meter and will reduce its output to try and get the export to zero. If there is still more power being exported when all of the RHI's PV power is being used to charge the batteries, the RHI will start charging "from grid". From grid in this case will be from the power that the additional inverter is generating.
So, what will happen is the house will be powered by 500W from the additional inverter, 500W from the additional inverter will be drawn by the RHI to charge batteries, together with its own 2000W. So you batteries will be charged by 2500W as you want.
Is there anything to be aware of regarding monitoring?
Up to you, either just monitor via CT clamps on the individual feeds or a more detailed RS485/modub as rpdom mentioned.
How do the two inverters get wired together?
The inverters will simply be AC-coupled both of them on the house side of your grid power meter. Obviously, you will need to ensure that the wiring and fusing between inverters and your CU is done to BS7671 standards and building regs Part-P, if applicable, and that they are sufficient for the additional power and that the fuses are chosen to correctly protect the wiring.
If the second inverter feeds into the first one (so it can charge the battery) does that affect first inverter's metrics (it feels like there might be double counting)?
Only, as detailed above, which is what you want.
Does the second inverter affect in any way the first inverter starting up or efficiency? I assume not, but sometimes these things go unmentioned.
No, they will act independently.
YW. The only other thing to consider is that you will need to get G99 approval from your DNO, prior to purchasing and installing your 2nd inverter. Depending on your local grid, your DNO, _may_ restrict the export you are allowed. This is not an issue as the RHI is already G100 export power scheme compliant, so you will just need to provide your DNO with the relevant G100 certs, diagrams of how it is configured etc, then set the RHI to limit its power to the value the DNO request.
