FoxESS Community - Owners & Installers Forum

I am having issues with two installers and would love advice please.

Company A installed 20 solar panels and quoted for an F series inverter as I had asked for the system to be ready to add batteries in the future.

On install, I noticed they had actually installed a G8 which they told me was a string inverter. When I queried why a different inverter had been put there, I asked if it was “better” (although I don’t know what that means, to be honest). I never got an answer. They did say that in the future, an AC charger could be added.

Firstly, is it true that adding an AC charger would allow for batteries?

Then along comes Company B. I told them I did not want the existing setup changing in any way, so as to not invalidate the warranty from Company A. They agreed to add another inverter and two 10kWh batteries.

The installer from Company B arrived and I checked that he understood we are adding and not replacing the inverter. He said there would be some efficiency loss with converting back and forth between DC and AC. I said I am willing to accept that loss in order to preserve the warranty and he mounted the extra inverter on the wall.

Does anyone know what the loss in efficiency would be?

Part way into the install, a lot of phone calls were going on. The electrician said to me that there would be a problem with having two 8kW inverters (New one is a KH8) as the DNO would class it as a 16kWh system. I said I would call the DNO myself to ask if this is a problem an if this is how it works and I repeatedly tried to get though. When I went outside to let him know that I was still trying, I found that they had already replaced the inverter, against my wishes.

Does anyone know if having 2 inverters would make a 16kW system? Or would they be inline and one would simply output/input at 8kW?

So, presently, I have issues with both companies, trying to work out who has told the truth/porkies, and what really should have been done.

Thank you so much in advance for your advice!

The F series wouldn't be big enough for your 20 panels, and the G series is effectively the next size up so that makes sense for your 20 panels ~ 8kWp

It's difficult to be precise about the DNO advice as we would need to know what your existing approval is for, if you have 8kw of panels, then i'm assuming you must have a G99 with an approved 8kW export limit but it might be different (and certainly wouldn't be for any more than 8kW).

The DNO would class the KH8 as a second inverter, and that would need to be approved for connection to the grid, that would take another G99 submission and it's very possible that your DNO would not permit it.

Technically there are ways you could link the 2 inverters together so the KH inverter does not exceed your agreed DNO limit but that would also need to be submitted to the DNO, 30 days in advance as it would constitute a material change in the system, and they still might not approve it.

What you have ended up with is a single KH8 inverter with your existing panels, and forgetting the warranty in my opinion is what you should have started with given you wanted to add batteries, and is the correct solution for where you are now.

A single inverter will be far more efficient than having 2 inverters, it's a hard one to answer as the losses are worse when the inverter isn't working as hard, plus a battery only inverter stays 'awake ' overnight and has a continuous standby drain that typically comes from the batteries when solar is not present, but your losses would likely be over 15% doing the DC/AC conversion from your first inverter and then a further AC/DC conversion to charge the batteries (i'll assume battery losses are zero even though they aren't, as they are similar for whichever system you have).

Thank you so much, Dave!

It seems like Company B should have known not to say that they would add an inverter and then go ahead with putting it on the wall and then changing their minds, leaving me with holes where it was for an hour or two!

Your advice is logical, and I feel that Company B should have known this and advised me accordingly that keeping my existing inverter was not a viable option in my situation. The were fully aware of my setup.

My export limit is 7.4kW.

Company B told me that they did not need prior permission to put the new inverter on - just that they would need to let the DNO at some point that it has been done.

Would adding an AC charger to the original G8 inverter have worked, rather than replacing the G8 with the KH8?