Hybrid inverter and Whole House Backup

Being new to the forum I am not sure where is the best place to ask this.

I am in the UK and currently have a standard 4KW system with 12 Sunpower Panels and a Fronius IG inverter which feeds to the single phase grid on a FIT agreement so this system cannot be changed and will not function in an off-grid setup.
I want to add another 5KW system with a separate Hybrid inverter and battery pack, firstly to balance the usage through the day and secondly so that I can have whole house backup in case of a grid failure with this new PV array also able to charge the batteries and provide house power when there is no grid.
I suspect that I may also have to have approved export limiting but this needs to be confirmed with the DNO.
I have identified a way of doing this with a Fronius Primo Gen24 Plus 5KW with BYD HVS 7.7KW battery, 14 x JA Solar 370W panels, Fronius Smart Meter and some form of grid protection although I am not entirely sure of the setup of this.
One disadvantage of this system seems to be that even with an ATS there is a delay of 60 seconds before the inverter kicks in if the grid fails, but is this a regulatory thing.
Also the potential cost of the grid protection as the only solutions that I have seen are the TeleHaase NA003-M64 and one from Enwitec which may do the job but do not seem to be widely available and I suspect are expensive and probably overkill. This makes me wonder if a manual switchover or simple contactor solution is a more economical way of doing this but I am not sure how this is cabled to work in both off-grid and grid tied modes.
I am keen to hear advice from any forum members who have experience of this type of setup or any other ways of doing this as I am struggling to get any installers to give constructive information, partticularly with UK regs.

Does not Tesla sell their Powerwall 2 in the UK? I've seen them in use there by video bloggers.

A regular Fronius (not hybrid) inverter + solar PV + PW2 installed with a Gateway would do what you want (technically at least), including UPS cutover to backup supply mode in the event of grid outage as well as take off-grid control of a 5kW Fronius inverter and PV system.

If you are not permitted to export from this second system, then the Fronius with the Fronius smart meter can manage that export limitation, while the battery will soak up what it can.

I guess the issue is understanding what it is you really want from the extra system? What is the primary objective?
Is it for financial benefit?
Grid backup?
Just want grid independence?

These are different objectives and achieving them requires different solutions. Financial benefit is very difficult to achieve with a grid tied battery system. They are just so hideously expensive that achieving payback within a reasonable time frame is unlikely. But they can be set up to be pretty seamless operations.

But since you are not permitted to export from an extra system, then why use a grid tied system at all?

Consider an off-grid system which can pass through grid supply as/when needed. IOW go off-grid and use the grid as backup rather than the other way round. The trick with this scenario is realising the off-grid inverter operates in series with the grid supply, not in parallel with it (like a grid-tied inverter does). IOW the supply is limited to the capacity of the inverter(s) you install and they do not back feed the grid at all.

We do have the Powerwall 2 but I was lead to believe that it would only charge while the grid was connected but maybe I am misinformed and it is just the Fronius IG that won't operate in an off-grid scenario but a more modern inverter would. Also I prefer open systems and Tesla isn't.
The Fronius Primo Gen24 plus appears to incorporate the functions of a regular PV inverter and PW2 in one package at a better price.

In my current system I use virtually all of the power so I envisage that the second system would from day 1 be exporting within my allowance and not hit the regulatory limit so in fact I would be paid for this, but export limit would still have to be installed. The current Feed In Tariff system must remain connected to the grid as I get paid for what I generate plus currently a deemed export of 50% however adding the new hardware will mean I will have to change to metered export.
Within the life of the whole system I will be changing to an electric car so I am trying to build a flexible system that is over capacity to begin with.

My primary objective is environmental but within a realistic price bracket and I have concerns over grid stability for the future. I live in an isolated location at altitude with a borehole so in a disaster scenario; we get a snowstorm that kills the power, the water stops flowing and ultimately freezes the water treatment plant. As we have the capability to generate power it makes sense to be able to use it in all eventualities, it just needs to be planned.

Please elaborate on the configuration of an off-grid series with grid supply. How does the grid provide the backup through the off-grid inverter? Wouldn't it then really need a 15KW inverter to match the 60A grid supply and wouldn't it be a bit expensive? I suppose in this system the existing PV setup would sit on the grid side of the off-grid inverter. It sounds rather more complex than I was thinking.

bleara said:

Please elaborate on the configuration of an off-grid series with grid supply. How does the grid provide the backup through the off-grid inverter?

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Imagine for a moment building an off-grid system with at its heart an "All-in-one" inverter. There are various brands and models of such inverters but let's leave that to one side. We'll get to the grid supply part a bit later.

You connect a solar PV array to the AIO inverter as well as a battery and the AIO inverter manages the charging of the battery as well as supplying AC output to loads from the PV and battery.

It will use PV supply in preference provided sufficient PV output is available, otherwise it will supplement the PV supply with supply from the battery, or of course use the battery solely if there is no solar power available.

Now here comes the neat part....

Most of these AIO inverters also have an AC input. And they can be operated in a mode where the AC output can be supplied by directly by passing through power from the AC input. That AC input could be from a (good) generator, or it could be grid supply.

IOW you could use this system to supply your home's power from the solar PV and battery, but should the the solar PV not be there and the battery's capacity drops below a set threshold well you can flip over to grid pass through mode to keep the power flowing, even top the battery back up if you wish to not wait until solar PV is producing again the next day.

bleara said:

Wouldn't it then really need a 15KW inverter to match the 60A grid supply and wouldn't it be a bit expensive?

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Well this is a choice you can make - there is no rule which says the system needs to supply power to all the circuits in your home.

You can scale it to cover most of the day to day consumption on regular circuits (e.g. general power outlets and lights), but exclude high power draw devices from the off-grid supply - and instead leave those connected to grid only (with their energy consumption offset by the grid tied solar PV system).

And if you set it up such that for whatever reason you may need/want to, you can flip a transfer switch back to having all circuits supplied by the grid directly and not be passed through the off-grid inverter.

Of course if you want such a system to have the capacity to manage a full 15kW draw, then you can certainly build one to that scale, usually by running two or more such AIO units in parallel and of course having the solar PV and battery scaled to suit.

The downside of an off-grid system like this is generally it does not feed back to the grid, although having said that, such options do exist with this capability.

bleara said:

We do have the Powerwall 2 but I was lead to believe that it would only charge while the grid was connected but maybe I am misinformed

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It depends on how it is configured. Combined with the Powerwall Gateway, in an outage the Gateway can automatically isolate the house's system from the grid and the PW2 can take complete off-grid control of the solar PV inverter's production via frequency control. It creates its own micro grid.

In this mode the system will supply household loads from PV and also be able to charge the battery if excess solar is available. The limitation is the power supply is restricted to whatever the PV and battery inverter's can supply. Unlikely you'll cover your full 15kW in this scenario either, that would require more inverters and batteries.

Normally in these set ups the system's owner specifies which circuits are to be excluded from receiving backup supply, and the Gateway is wired accordingly. If for some reason the power demand exceeds the system's capacity, then the system will shut down and require restarting, with the offending loads powered off.

There are limits to the size of the PV system you can connect in this manner, ~7kW per PW2, that's so the PW has enough time to manage the potential inrush of current should the grid go down while the solar PV system is pumping out/exporting at max capacity.

Since in your case the export limitation is not as severe as I first thought, then the first preference would be to keep both the next PV system and battery as grid tied but with the system designed to run as an isolated micro grid during grid outages. This however is not cheap, while an off-grid system with a transfer switch can be put together for a whole lot less money and do 95% of the job.

wattmatters said:

Imagine for a moment building an off-grid system with at its heart an "All-in-one" inverter. There are various brands and models of such inverters but let's leave that to one side. We'll get to the grid supply part a bit later.

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I think I started with this idea a year ago after my mind integrated that a grid-tie inverter merely needed a 240v1p space on my panel. Check! My house has a 225a panel and with the exception of inductive loads, 5kva backfeed from the AIO would be more than sufficient. And I didn't mind oversizing the AIO to handle the HVAC and/or disable it through smart home while not on bypass.

But then I realized most ESS is different. AIO isn't just a bigger grid tie, there are a lot of safety considerations such as quick disconnect that keeps one's AIO from shunting out as it powers the rest of the neighborhood through the panel and back out the mains in a blackout.

wattmatters said:

Well this is a choice you can make - there is no rule which says the system needs to supply power to all the circuits in your home.

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My desire for this is less about "whole home" and more about the convenience of not trying to sort panels. If there's a load in the house, it goes to the panel. Loss of grid would trigger smart home to immediately turn off (but not disable) all loads and put HVAC into emergency mode (above freezing or below hyperthermia temps, depending). Lights and appliances would keep working, but manual control. Turn off the hot tub, but keep it from freezing. And when the grid comes back, restore everything to normal (unless the cellphone is in DND/wakeup alarm set, which means "don't turn the lights back on...")

It's so interesting how many goals there are for different users!!

I was originally looking at an Outback Skybox as da bomb, but realized it depends on a subpanel. Sol-Ark works without one, but it seems to have a prohibition on AFCI/GFCI breakers, probably because the output is noisy and the breakers would trip.

Any ideas here?

Orangesauce said:

Any ideas here?

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I'm not an electrical engineer or electrician so some of the possible scenarios would best be described by others.

But I can describe the set up I have which is one option to achieve what you are after.

I have a grid tied PV system and also an off-grid PV+battery system. The primary task of my off-grid system is to maintain a battery bank at full charge ready to supply backup power if/when needed. It also runs a pool pump from the available spare PV capacity but that's a side benefit.

In my main circuit board I have a transfer switch which on the backup side is fed power from a power inlet. The AC output of the off-grid system is connected to the power inlet, in the same way you might connect a generator for backup supply.

If the grid goes down, then the transfer switch is flipped over to the backup position and power is then supplied to the home's circuits from the off grid system. When operating with the transfer switch in the backup position all circuits connected to the backup side of the transfer switch are now completely isolated from the grid. It's an isolated system and no back feed is possible.

In my main circuit board selected circuits are excluded from the backup side of the transfer switch. These circuits are left connected to the grid supply in a normal manner and hence only receive power if the grid is active. These include, inter alia, my grid tied inverter, large ducted aircon system, oven and induction stove. The grid tied system shuts down if grid power goes off. It cannot operate without a grid signal and so it cannot back feed either when the grid is out.

Now while my transfer switch is manual, it could be an switch which automatically cuts over to backup if a grid power fault is detected.

In my case that's not really necessary as I have UPS units on key devices I want to not suffer from even short power interruptions, and it gives time to do the manual switch to back up.

But the off-grid AIO inverter has another trick up its sleeve - Utility First mode. In this mode, like I mentioned earlier, it passes grid power supply through the AIO inverter directly to the output.

So if the grid is online AND the AIO inverter is in Utility First mode AND the transfer switch is in the backup position, then all the circuits in the home are powered by the grid. It's just the selected backup circuits get power via a slightly more circuitous path (grid -> AIO inverter -> transfer switch -> backup circuit).

However, if the grid goes down then the AIO instantly cuts over to supply power from the battery (and PV if available). It operates just a like big UPS. Meanwhile, the circuits which are not connected to the backup side of the transfer switch lose power.

When the grid power returns, the non-backed up circuits have power again, and the AIO inverter will revert back to passing grid power though to the backed up circuits.

I had this very scenario the other day. This is a chart showing power from my off-grid AIO inverter.

The blue line is the AC power output from the inverter - this is what is being supplied to the backed up circuits from the inverter.
The red line is the grid supply to the AIO inverter.
The yellow line is the PV power supply (not much as this is mostly overnight)



- it's late afternoon and storms are hitting our area
- Just before 5pm the grid goes down
- I flip over the transfer switch and the off-grid system immediately supplies power to all backed up circuits.
- I also, remotely via my phone, change the operating mode of the AIO inverter from its normal day to day Solar/Battery mode to Utility First mode. This doesn't change anything at that moment since there is no grid AC input to the AIO inverter, so it continues to supply power from the battery.
- About an hour after the grid goes down, the grid power returns. I know this because the oven clock comes back on and makes a bleeping sound on power up. Since the grid power returns the AIO inverter reverts to passing through the grid power and ceases to draw power from the battery.
- because it was night time and it's not unusual for the grid to go offline multiple times on nights like this, I decide to leave the transfer switch in the backup position and the AIO inverter in Utility First mode.
- as it turned out the power went off and came back on three more times that night. Each time the AIO inverter seamlessly kept power supplied to the backup circuits, while those grid only circuits would lose power.

You'll note that when grid power is passed through, the grid power is a little higher than the output supply power. That's because I also set the AIO inverter to recharge the battery from grid supply. It can be set to not recharge from grid but I figure it may as well keep the battery topped up because who knows when and for how long an outage might last.

That's great, thank you for taking the time to share that much detail in your posts!

I live in a mountain state where the grid is spotty in many areas. I live (for now) in a suburban area with relatively new grid infra, but have started watching the power lines when I travel regionally. It's incredibly interesting to trace the different grid capacities by the number of wires on the poles and the types of insulators that the wires are suspended with. I always considered the two extremes of on-grid / off-grid, but never thought through the nuances of a weak grid, even at the same time I have imagined exactly these environments while travelling. A single conductor on a pole in the middle of nowhere is just some number of houses crying in pain when that point of failure finds it's limit. You've distilled it down to a pretty rote process. Well done, and thanks for the education.

What I am processing over the last few days is how much I want to invest in this home. The grid here is highly reliable and stable and the home wiring is NEC 2011 to what I can tell. There are some aspects to the home that I love and non-technical issues that surround the home that are unbearable. So another option would be going more outback in an area that isn't quite so cookie cutter and applying my experience in a more demanding utility environment like yours. Especially on a new home designed for this kind of thing with a battery bunker for the explody stuff...