Can this be done ?

This is an interesting question, and as usual the answer is yes and no. As @meandershas explained, the output of an ordinary inverter can not be mixed with the house mains supplied by the national grid. The AC waveform would need to match exactly in voltage, frequency and phase. An ordinary inverter has no mechanism to match the waveforms, so they can't be mixed.

You have the option of running some house loads via an extension lead from the MH. Possibly with a changeover switch, either manual or automatic, so that the house mains can take over if the batteries go flat.

The inverter you linked to in post#10 is a 'grid-tied' inverter. It is permanently connected to the mains, and takes its waveform from the incoming mains. It 'inverts' the DC power coming from the solar panels. No batteries are involved at any point.

If the grid power fails for any reason, the inverter goes off as well - that's something that's not in the advert. It produces whatever power it can from the solar panels connected to it. You can use that power in the same way as the grid power, and any excess power not used gets fed into the grid.

The Victron Multiplus inverter/charger mentioned in other posts is a different beast entirely. It connects to batteries and can produce 230V AC just like a standard inverter. However if a grid connection is made to it, it can synchronise exactly to the grid waveform, and mix its output with the grid power. It has various operating modes, one of which is to limit the input from a low-amps hookup post and add its own output so that high power appliances can be used. So for example a 6A hookup will normally only supply 6 x 230 = 1380W, but with a Multiplus you could run a 2200W kettle from it, as the difference would be supplied from the batteries via the Multiplus inverter.

But that's just the start of its capabilities. If you like tinkering, the Multiplus input and output can be controlled by a controller like the Cerbo GX, or Venus OS software running on a Raspberry Pi. The Multiplus accepts Modbus commands for input and output. These commands can be sent by a Raspberry Pi script. Or if you use the 'large' version of Venus OS, you can use a NodeRed program. You can program it to do all kinds of things, including exactly what you describe - using battery power to generate mains power to run house appliances within the house. You will need to add some current-measuring sensors clipped round the mains wires, with data transmitted by radio (Zigbee). But yes, entirely doable with a Multiplus.
 
You still need ESS assistant, and a country grid code. Zigbee is to slow for the sensor, hard wired Carlo gavazzi ET sensor on RS485 is needed, or the newer digital victron sensor, but definitely not wireless, it will react way to slow, and render the current sensor useless. It needs to respond within milliseconds, in order for the multiplus to react and modulate it’s power with the loads.
 
You still need ESS assistant, and a country grid code. Zigbee is to slow for the sensor, hard wired Carlo gavazzi ET sensor on RS485 is needed, or the newer digital victron sensor, but definitely not wireless, it will react way to slow, and render the current sensor useless. It needs to respond within milliseconds, in order for the multiplus to react and modulate it’s power with the loads.
I haven't actually tried this, so that's good to know, one less dead end to avoid. The ESS assistant is an Energy Storage System app that runs on the Multiplus. You could use this if for example the Multiplus was installed in the house or maybe the garage. But it explicitly says it can't be used for mobile applications. I'm looking at finding a way round this problem, maybe with a custom control app similar to the ESS app, but with a switch so that it can swap between a motorhome system and an ESS-type app.

I have about 7kWh (600Ah) of lithium and a Multiplus 3000 in the MH, and it would be nice to use it as a proper battery storage system when it's parked on the drive by the house. If the battery and Multiplus were just installed permanently in the house there would be no problem, it would work fine as an ESS. There's about 4kW of solar on the house roof, and a separate grid-tied inverter processing the power into mains.
 
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I haven't actually tried this, so that's good to know, one less dead end to avoid. The ESS assistant is an Energy Storage System app that runs on the Multiplus. You could use this if for example the Multiplus was installed in the house or maybe the garage. But it explicitly says it can't be used for mobile applications. I'm looking at finding a way round this problem, maybe with a custom control app similar to the ESS app, but with a switch so that it can swap between a motorhome system and an ESS-type app.

I have about 7kWh (600Ah) of lithium and a Multiplus 3000 in the MH, and it would be nice to use it as a proper battery storage system when it's parked on the drive by the house. If the battery and Multiplus were just installed permanently in the house there would be no problem, it would work fine as an ESS. There's about 4kW of solar on the house roof, and a separate grid-tied inverter processing the power into mains.
In a mobile install you are correct as not to use ESS. I whole heartily recommend virtual switch, with couple of commands; ignore AC in, and Flag to notify it when to resume. You will used loads on the AC out side, with the ability to have grid on AC in connected, synchronised, but ignored until needed to contribute. A match made in heaven for self consumption with help from grid to support those bigger loads.
Look at Adam de Lay channel, that gentlemen has dedicated his time in explaining this functions in detail.
I also successfully installed and set 2 multipluses in parallel on a friends mountain cabin. Same principle, virtual switch, ignore AC, and only allow power from grid to flow in pass trough when load exceeds 8kw for 5sec. Then grid ignored again when power is bellow 4kw for 10 sec. works a treat as grid is synchronised and ready to contribute. This allows to use all your production, and only supplement from the grid when necessary. Also this way, the hardware makes it impossible and is guaranteed 0 back feed where not allowed. If you need any pointers on programming the virtual switch, let me know.

Just seen your edit, there is no easy way of utilising your Moho multi in parallel with the house grid. Separate yes. Even if you are allowed to export, you are risking export at night time from the multi battery. Needs another multi on the house, programmed as I described above. It will take the power from the Moho multi into the house grid parallel.
 
Hi Paul

Personally I’d just run an extension lead from the Moho

Always keep it KISS 😊

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