@transparent Thanks for those interesting posts. I had read about the importance of 50Hz synchronisation in one of your earlier posts in other threads.

Living in a rural location, off-grid operation would be handy in case of power cuts, and, as the house is in the process of renovation, it would be easy to run some additional circuits to provide back up power (AC) to sockets purely for off-grid use.

I have wondered about running heating directly off the battery but there don't seem to be any off the shelf electric heaters suitable for DC. I have seen YouTube videos of people converting AC heaters to work on DC but they seem to be a bit of a bodge and are probably tempting fate.

@transparent Thanks for that info. I shall try to keep the setup as simple as possible initially and maybe later, as more battery capacity is introduced, add additional operational flexibility. That will probably cost me more in the long run but I don't want to take on too much all at once. It also fits my time constraints.

Posted by: @transparent

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As you'd expect me to, I've just checked the spec of that SPH5000 inverter @majordennisbloodnok

Let's note that the off-grid (backup) mains output isn't rated at the full 5kW of the inverter.

It's actually constrained by the discharge current available from the storage battery.

In many cases that wouldn't matter.
If I needed high output to power a larger load, such as a heat-pump, then I'd buy a second inverter and connect them in parallel (as per my diagram above).

But the Growatt SPH range of inverters don't have have a parallel-operation option.
If you later decided that you required more power then the SPH would need replacing.

I thought I'd better mention that before @homonid realises how cheap the SPH-5000 is and orders one.
(He's already responded above before I just posted this warning!)

I have to say I would neither recommend nor warn against the inverter I have. My wife and I got it as part of a Solar Together bulk buy initiative in conjunction with our local county council and so we had no influence in which manufacturer's kit would be touted. The savings were enough, though, for us to go with it anyway.

My experience is that the hardware has done what it's supposed to perfectly solidly, but that the app and web offering from the manufacturer are rather less polished than I would like. That was my main driver for starting to use Home Assistant to communicate directly with the inverter and sidestep all the web-based hassle. The combination of the inverter itself and some form of third party locally-connected management system is something I would be far more likely to recommend although I have seen friends with different brands of inverters that have actively impressed me.

You don't need to run separate/different mains circuits for off-grid operation @homonid  

I simply have on-grid and off-grid consumer units next to each other.
An electrician will connect the wires for each circuit to whichever box you tell him to!

I've got a friendly electrician who will work alongside me for my own renovation project.

I put backing boxes in the walls and run the cables between them.
He then connects the mains sockets and does the continuity and insulation checks.
That means I get a full Part-P certificate and assurance that the work is up to the required standards.

An electrician will typically quote for a job by totaling up the number of mains sockets and multiplying by £100.
But the great majority of the work doesn't require his (expensive) skill set.

I can buy a backing box and quality double-outlet 13A faceplate for less than £5, and 2.5mm² cable for £72 per 100m drum.
So I can have many more socket outlets than a professional would usually install.

Just make sure that all the wires come back to a common location in the home.
When you later add more inverters/batteries, the wires for more of your existing circuits can be moved to the off-grid distribution board.

If I really can't make up my mind, there's also some double-pole changeover trips which can be installed in a consumer unit:

Note how I have copious labeling.
Everything needs to make sense to any electrician who may need to work on site in the future.

Posted by: @homonid

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I have wondered about running heating directly off the battery but there don't seem to be any off the shelf electric heaters suitable for DC.

Low voltage DC heating plates are used in the pet sector by those who keep exotic reptiles in a vivarium.

You don't want to risk electrocuting an entire colony of anoles when one of them chews through a cable!

But I think we'd like to know a bit more about your overall aspirations for heating the home @homonid

If it's a renovation project and under your control, why wouldn't you install underfloor heating pipes?

When I bought this property in 2024 the oil fired boiler was on its last legs so I had installed a 12kw ASHP supplying new, oversized radiators. The house had not been modernised since the 70s and being grade 2 listed had no double glazing, and being stone built, not cavity walls. Thus insulation was very poor and the ASHP was running up huge bills.

Insulation improvements have been made and more are on the way but it is still well below what a modern home should have and the bills are still crippling. Hence the desire to save cheap energy overnight in a battery bank to use during the day.

Living in Cumbria where temperatures can stay below zero for a couple of weeks the ASHP if kept on 24/7 can use 50 units a day. I hope that once fully insulated that will come down to half that figure. If that is the case then 30kw of battery back up would give me very manageable heating bills.

I have been doing my own electrical installations which are then tested and connected by a friendly electrician and am looking forward to the challenge of setting up a home battery supply.

@homonid, there are a couple of other threads that might well be of interest to you.

(Controversy Warning) Is it time to relax Listed Status for Insulation purposes? – Insulation – Renewable Heating Hub Forums

Installing a heat pump in a Grade II listed property – Air Source Heat Pumps (ASHPs) – Renewable Heating Hub Forums

Although the second has started down a bit of a tangent now - although the discussion is interesting nonetheless - both of these threads are discussing many of the issues you're facing so are worth a read. I don't doubt you'll already know much of it and may well be able to contribute to them but since the subject of heat pumps in old buildings has been discussed before I'd be remiss not to point you towards them.

Thanks for those links, always worth taking a look.

Posted by: @homonid

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I had installed a 12kw ASHP supplying new, oversized radiators

Thanks for that information....

This affects the recommendations for inverter capacity.

Whilst a typical 12kW ASHP might use 10A whilst running, it's likely to draw at least 30A when starting.
That's because the compressor and the pump have to raise the pressure of the fluid loops on both sides of the heat exchanger.

So this isn't just the switch-on surge of a motor starting.
The high-current draw continues until the control electronics detects that operational pressures and flow have been reached.

Obviously a heat-pump should be configured so that it never cycles on/off.
But your inverter(s) still need to be able to start it if/when required.

There's a fair amount of physics behind those comments, but let's try to avoid plunging too deep into a technical hole.

Give us the model of the ASHP and I'll look up the specs.

@transparent One day I hope to have enough battery storage so that the heat pump can be left on 24 hours, but until I get the house fully insulated the heat is just going out the windows, so I only use it overnight on the cheap rate.

The heat pump is a Nibe F2040-12kW.

The devil is always in the detail.

Thanks.

Just a thought, two companies have tried to install a smart meter in my property, one only just last week, but both failed to get a signal due to hills and a large barn in the way.

I assume that with no smart meter I cannot export to the grid as the utility provider would have no way of measuring what was exported. Do you know if that is correct?

Thanks.

Smart Metering is one of my specialist subjects @homonid  😎 

It's possible that your home is one of the 320,000 sites where it is unlikely that a Smart Meter can gain access to the SMWAN (the national network).
DESNZ (Dept of Energy) and the SEC (the Smart Energy Code governance organisation) have recently undertaken a consultation on a proposal to overcome this deficiency.
There were just 24 respondents, including myself.

The interim 'report' from DESNZ is 'white-listed' and you can read it here.

You fall within the bracket of 'interested parties' and you may refer to this report in communication with your Energy Supplier.

I'm somewhat puzzled that an engineer should actually arrive on site with the equipment.
When the job is allocated to an installer, they search the National Coverage Database to ensure that they bring the required model of Communications Hub with them.

If the site is beyond the scope of an installation team, they contact the Data Communications Company (DCC).
DCC have additional resources including high-gain external aerials and engineers who can be sent to site.
So the correct procedure should be that your Supplier refers you onward rather than just gives up.

A little known feature of Smart Meters is that the SKU2 and SKU3 Comms Hubs have an integral MESH Network feature.

In a rural area, any four or more Smart Meters will automatically form a Mesh.
Data and Commands are passed along chains of meters until they reach one which does connect to the SMWAN.

My own house on a hilltop acts as a Mesh Relay point for homes in the valleys to the West and North of my site.
I have a SKU2 Comms Hub with an external aerial to achieve this.

It's possible that your location might yet be able to have a working Smart Meter connection if more of your neighbours also had them installed.

You need to raise this situation using the Complaints Procedure of your Supplier.
That gives them a pre-defined timetable for responses.

Always use email when handling a complaint.
The date/time stamps would be required if you end up taking the case to the Office of the Energy Ombudsman.