Regs wise there is nothing stopping a second Ring or Radial circuit being connected to another supply, so long as things are clearly labelled and it is obvious how and where to isolate it if need be.
However it would be most unusual, and may be a bit confusing for a non- technical user if some sockets were live or dead depending on when the sun last came out. Normally UK practice is that the inverters are "grid tied" that is to say they connect to the house mains, and when generating push current back in the opposite sense to a normal load, so the voltage rises rather than falls  moving from substation to house. That way the socket has a supply all the time, but uses the inverter when available.

Thinks to watch -

1) earthing systems will need to be co-ordinated (i.e. solidly connected) so that there is no chance of a user straddling the two systems getting a shock from the cases of one appliance plugged into each.Also, as parts of the system may be live even when the external mains is off, then the external supply  earth cannot be relied upon as the sole means of earthing. -the external earth may be disconnected while it is being repaired.

2) The performance of the inverter under overload or fault will be very different to the mains supply, and may not be enough to reliably trip a normal breaker. This may mean a non0standard arrangement is needed.

Can I ask why like this as opposed to the more conventional ?
Mike.

I'd add that many kitchen appliances have starting currents significantly higher than their nominal ratings - it's not just anything with motors or electronic PSU, even simple resistive loads like heating elements will have a much lower resistance when cold. Inverters often struggle to supply such starting currents - and may have to be very significantly over-sized to cope (along with the d.c. side to match). Grid-tied systems avoid such problems as the grid will automatically make up any shortfall.

  - Andy.

hello  thanks for consideration :  You underline the two potential  show stoppers Labelling / Earthing : Grateful advice 1 and 2 below 

Ok prelim  reasoning for  modified " standby generator " approach :Ponder if trad inverter grid tie is a bit of a scam? If your not there when the sun shines  Its all to waste ? (Discounting the  now miniscule pay back for regen to grid in off peak times)  .   So makes sense to me: Power when you want it  At point of use .  Yes accept  unaware folks could be confused with unpowered sockets  Adjustment to off grid living ? Outlook global warming> fossil fuels  looks bleak ?  There is always a prominent visual  display of battery status to indicate. .  But yes also devising a timed signal warning for connecting fridge  in case you have forgotten ( LOL) 

Most grateful for advice 

1. labelling  identifying  sockets  Propose make them all single  sockets maybe surface mounted Red paint of corner  Notice at visual display of battery status to effect Red sockets have alt supply Isolate at ( invert3er in garage )  Yes am going for the under counter 

Not very elegant Any suggestions  

2. Earthing   DC cable solar array will have earth conductor ( mountings) as will inverter power cable supply  The house is TN=S   Should  we should connect the solar installation main earth to the house earth point ? Not separate ? 

thanks again Ms O 

thank you andy yes accounted for : see my reply to other |: Grid tie uncertain future .Grateful answers to the two unaccounted regs items 

Labelling and earthing , Ms O  

yes solar earth and incoming mains earth should be solidly connected, but in addition you will need an earth electrode(s) of your own, so that all earthing is not lost if for example the external power cable is disconnected during repairs in the street.

Mike.

Anne Otis said:

Ok prelim  reasoning for  modified " standby generator " approach :Ponder if trad inverter grid tie is a bit of a scam? If your not there when the sun shines  Its all to waste ? (Discounting the  now miniscule pay back for regen to grid in off peak times)  .   So makes sense to me: Power when you want it  At point of use .  Yes accept  unaware folks could be confused with unpowered sockets  Adjustment to off grid living ? Outlook global warming> fossil fuels  looks bleak ?  There is always a prominent visual  display of battery status to indicate. .  But yes also devising a timed signal warning for connecting fridge  in case you have forgotten ( LOL) 

Rather having a parallel off-grid arrangement, have you considered grid tied batteries, with a controller that charges the battery or switches on dispatchable loads (e.g. immersion heater) when there's excess solar and discharges the battery to feed loads there's not. Many inverter manufacturers provide off-the-shelf solutions for domestic applications, with a likely reduction in both equipment and design costs... Most solar installers these days will have a preferred answer that they've used before.

No need to duplicate circuits or source DC equipment or having to switch sockets over when the batteries are drained, and you retain the benefit of grid support for starting currents etc so somewhat simpler for the user and maintainer. Does mean you can't run in the absence of grid voltage (island mode systems are available but somewhat more pricey), but it doesn't sound like you're too worried about that (and sizing batteries for a reasonable autonomy time will be a big undertaking whichever way you do it).

The house is TN=S

It might appear to be TN-S, but if you're on the British Mainland, chances are if you were to ask the DNO you'd be told that 'PME conditions apply'. The problem being that for decades now the DNOs have been regularly converting separate N and PE conductors in the street to PEN whenever they do any replacement or repair work (even short lengths mid way along an existing cable) - so your supply carries all the risks associated with PME (TN-C-S) even the combined N/PE conductor doesn't quite reach your cut-out.

   - Andy.

Ponder if trad inverter grid tie is a bit of a scam? If your not there when the sun shines  Its all to waste ?

You've got that problem either way with PV (although the newer smart export tariffs generally pay significantly better per kWh than the export side of the old FIT tariff). Adding a battery helps of course, but grid-tied battery system can be setup minimise imports from the grid - so everything is supplied from the battery just as long as there is charge available and up to the capacity of the inverter - but if the battery is flat or the load exceeds the inverter's output, then the grid automatically makes up the difference. Same result I think, but a lot less hassle.

   - Andy.

To throw in my thoughts:

My grid-tied system has solar and a battery.  The inverter is reasonably clever in what it does with the solar power during the day, prioritised as follows:

1. Match the load I'm using in the house (highest priority)
2. Charge the battery
3. Export anything left over

An off-grid inverter would lose option 3.  So even if the electricity companies only pay pennies per unit of exported electricity, it's still free money when compared with not exporting it.

The system seems to be able to generate up to 3.4kW from solar in ideal conditions, or about 2.1kW from the battery (it's a very small battery).  So much of the time, it's powering the entire house, and not only a few selected appliances.

If there is a power cut, I can operate a big switch to go over to "islanded" (off grid) mode.  That disconnects the whole house from the grid, and re-connects it to a second emergency output from the inverter.

And Simon's sort of system is much easier to install - no trick wiring of non mains circuits, easier to use (no forgetting to move the freezer to the 'other' supply) and still allows Island operation.

I have a work colleague with a similar system and also  quite a big battery (upright freezer sort of sized thing) and even on no-sun days he has made a profit by charging the batteries off the mains at cheap rate and selling it back to the grid at peak time.

Being an electronics sort of chap it is slightly more complex than normal in that the contactors that  switch to island operate automatically, and there are two consumer units, one for 'backed up' stuff that runs island mode when required, and the other for loads that get dropped if external power is lost. It was I think rather pricier than the minimal offering.

Mike.