Combined on/off-grid PV backup power

[mikepepler]

Following on from an earlier thread, I realised that if I took my existing 280W PV system off the roof (and resold it of course) then I would have space to fit 3 or 4 250W panels. This would have a few implications:
- they would not be 12V compatible without a suitable charge controller, losing some redundancy. (Having said that, a suitable charge controller could be bought, or a 24V system used)
- they would need to be grid connected, as there would be more power than we could sensibly use on some days.
- To keep an off-grid capability, I'd need some method and equipment to disconnect the house from the grid and run it from the panels/batteries.

I've seen rather expensive systems that will do this, e.g. AN003 at this link. But this requires a £1,000 box of tricks in addition to the other hardware, and a separate circuit in the house that's run via backup power. I'm not interested in rewiring the house, and am not worried about needing to manually switch over to backup power (i.e. I don't need UPS functionality).

Has anyone on here actually got a grid-connected PV system that can also operate off-grid? Theory is all very well, but I'm interested to know if anyone's actually done it and what it cost?

[adam2]

I have not actually done it, but it should be achievable at relatively modest cost.

Connect PV modules to a grid tie inverter in the usual way.
Install a suitable battery bank, and a single PV module with charge controller to keep this battery charged, and to recharge it after any small or infrequent discharge.

After any short term power cut either wait for the single module to charge the battery bank, or use a grid powered battery charger.

In the event of any prolonged power cut or frequent rota power cuts, then reconnect the otherwise grid tied PV modules for battery charging.
The exact switching arrangements would depend upon the relative voltages.
A MPPT charge controller that accepts a relatively high input voltage will be needed.
For example if your grid tied array operates at 72 volts and the MPPT controller will accept this voltage, then a simple double pole double throw switch will suffice.
If the grid tied array operates at too high a voltage, then in battery charging mode it must be divided into sections each of suitable voltage.

[mikepepler]

Thanks Adam. WHat about how the power is distributed into the house though? My current system simply has a couple of extension leads that run out from the inverters to the things that I run off-grid. If I went for something that could work on/off grid, I'd ideally want the power to go through the mains throughout the house even during a power cut. I know this would need some extra hardware, but should also allow the PV grid inverter to keep feeding in, synchronised with the battery inverter/charger rather than the mains.

[mikepepler]

What about this situation:
- normal grid connected PV
- separate system that takes the whole house off-grid and runs it from a battery and inverter/charger.

Two questions immediately arise:
- what is required to be legal and safe, to make sure it is impossible to accidentally have the grid and the inverter charger trying to feed the houses at once?
- how would the system behave when off-grid with full batteries, no demand and high PV output?

[adam2]

Unless you are going to obtain an off grid inverter of most improbable capacity, you wont be able to feed the whole house from it.

A better approach is to install one or two circuits for the most important loads and fit manual or possibly automatic changeover so as to supply these circuits from the grid normaly with the off grid inverter as standby.

Depending on the size of the inverter and on your needs, I would consider connecting all the lighting circuits to the changover system, and one dedicated power circuit.
This power circuit could reasonably supply one socket in each room, a single 20 amp radial circuit should be ample since that is over 4KW and I presume that your existing or proposed inverter does not exceed 4KW.

A grid tie inverter is only designed to connect to the grid and I would advise very strongly indeed against attempt to try and interconect a grid tie inverter and any backup supply.

During a short or rare power cut this means that the potential output of the grid tie installation is wasted. This matters little in the short term.
In the event of a longer term power cut then the grid tied PV array, but NOT THE INVERTER may be used for battery charging as earlier described.

A simple double pole changover switch will be suitable for changing over between off grid inverter and the normal grid supply.

Do not worry about the grid tie inverter, they are required to shut down automaticly on loss of the grid. Just make certain that the grid tie inverter is connected to a "grid only" circuit and NOT to the dedicated circuit intended for backup power. The grid tie inverter will then shut down in a power cut. It wont "know" that other circuits in the building are on a backup supply.

[mikepepler]

Thanks Adam! I have a friendly electrician nearby, so I can ask him about costs for putting in a switch for the lighting circuit...

[adam2]

Yes, I would have your electrician proceed as follows.

Install a new small consumer unit, probably a 4 circuit one.
Remove from the existing consumer unit, the existing lighting circuits, connecting them instead into the new C/U
Install a new additional 20 amp radial radial circuit, feeding one twin socket outlet in each room, connect this to the new C/U

The supply into the new C/U should be from a double pole changeover switch in order that it may be supplied from the grid (via a 32 amp or 40 amp MCB in the original C/U) or from the existing/proposed inverter.

The new small C/U should be protected by an RCD, or alternatively each outgoing subcircuit should be fitted with an RCBO

The earth terminal of the new C/U should be connected to the installation main earth terminal.
A suitable earth rod should also be connected to the main earth terminal, if not already existing.

[mikepepler]

That sounds like a simpler solution Adam. We don't actually have any *essential* high-current appliances on the ring mains, just kettle (2.5kW) microwave (1.2kW), toaster (0.6kW), washer (variable, I think 1.2kW peak). Then stuff like a vacuum cleaner, of course. But all of that is stuff we would not use during a power cut anyway.

We've got two consumer units at present, each fed directly from the main supply. One smaller one does upstairs and downstairs ring mains, the larger unit does everything else, and has some spare slots in it. As the small CU is full, I guess that would come out and be replaced by a new one with space to add the lighting, and have the changeover switch control it.

But this will all come later... I've just bought two new surplus 250W PV panels for £240! These will fit on my existing roof rails, replacing 6x40W panels - the 7th 40W panel I have will stay there, feeding into a separate 12V system. So I'm going to stay off grid for now - I got a quote for a 1kW grid system and it was many times more than what I'll spend getting this 500W system up and running.

Once I have the new panels up, and the charge controller upgraded to handle them, then the next step will be to look at getting a large inverter that would be able to handle all lighting plus all sockets (assuming toaster, kettle, microwave, etc. are banned while off-grid). Then finally I can take a look at the wiring/CU changes...