I have some additional solar panels to my existing on grid installation, which I'm intending to use with batteries & inverter to run some of the household circuits off grid

I've already tested & have plenty adequate power, my Q is this, the inverter has a standard 3 pin socket outlet which I've been running things from whilst testing, if I now remove set circuits from the mains supply & feed them from the inverter, can the inverter & existing mains installation share earth & neutral connections?

I would assume sharing the earth would be sensible as otherwise anything running from the inverter isn't truly earthed, but is it safe/desirable to "share" the neutral with the existing installation, it would obviously make the connection simpler, but I have no idea if this would cause problems with the 2 separate AC supplies at slightly different voltages & presumably, different phases?

Any info much appreciated

I'm not a domestic electrician, though do have some qualifcations, but have no idea what the current regs are.
For educational/interest purposes, I'd say in theory you could, bearing in mind earth is earth and will connect together ultimately
However best practice I suspect would advise against it to ensure that there is no possibility of the two separate live circuits being connected for exactly the reasons you add.?
Anyone else?

I've been considering something like this myself for a 2022 project (probably to power an immersion heater and a couple of electric UFH set ups that we have) I've only just started think about the details.

I presume it'd be best practise (or required?) to feed your solar circuits through a mini consumer unit so that you have RCD protection from faults on those circuits? I wonder how you can protect against L/N to earth faults to things like central heating pipes unless you had earth bonding from those pipes to your solar consumer unit earth busbar? This would then leave a single common earth across all of the house as you suggest in your question, so feels unavoidable.

As you already have a grid-tied set up, why don't you intend to connect the panels up to the existing set up and then find a way of only charging the batteries when you have a power surplus?

I have a temp setup to try everything out.

The reason for not simnply extending the existing grid tied setup is that you're not allowed to it's already the max 4kWh & on the original tariff, so very lucrative! I also have a bit of kit that recognises whever you have excess power & are returning it to the frid, at which point it diverts that power & dumps it into an immersion heater, we get free hot water for at least 8 months of the year & even today it briefly had sufficient to put a little power into the cylinder! (but only because I now have some stuff running from the new panels)

I'm sure you're correct that anything from the inverter should be run thro' a mini consumer unit, for the time being I simply have a couple of the trip switches (RCD's?) from a consumer unit to control the lives & all neutral are shared with a common connector block which is separate to the existing mains neutral rail. I've shared the earth to existing earths at the consumer unit -all seems to be working happily at present

Let me know what you discover - it sounds like we'll be trying to solve the same problem really.

Revival of an old thread, just to say, you keep inverter live & neutral separate from the household stuff at all times.

Strictly speaking you shouldn't share the earth as technically it's owned by your DNO, however sharing it doesn't cause any issues unless you're going to have a major drastic fault. That said, as long as the system is protected by an RCD it is almost impossible to create circumstances where it could cause any danger to anyone