I have installed a Fox 8Kwh hybrid inverter with a 20Kw battery cube tied to 8Kwh solar panels to generate and store power to my home. The system is set up on self use as off grid and working well.
I wish to add a generator backup for when the solar generation is insufficient to restore the battery charge sufficiently before nightfall. Inputting the generator feed into the grid line what menu settings are required to ensure there is no backflow from the inverter (feed in) once the batteries attain their designated charge level. Can I just set the feed in power to zero in the setting on-grid menu?
Off grid setup with Generator backup supply
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Dave Foster
- Posts: 1558
- Joined: Thu Oct 13, 2022 7:21 pm
I think the best way to do what you want is to connect the generator to your consumer unit/fuse box on the load side of the main CT meter - then add a second CT clamp from the CT2 input, and clamp it to the generator live feed.
The inverter will see everything the generator produces as external generation (as if it were another solar inverter) - any excess above the house load will be used to charge the batteries automatically no settings required as long as the generators regulation is good enough - when the batteries are full the inverter won’t attempt to use the batteries as long as the generator output covers the house load.
Unless you are running totally off-grid i.e. no connection to the any grid - in which case you will be running in EPS mode - the problem then is that anything that you connect to the grid input will force the inverter to switch back from EPS cutting off your off-grid supplies.
In which case you would need to set the EPS circuit so that it is live when the grid is active i.e. you may need to change the setting on bypass relay and turn UPS on - then connect the generator to the grid input.
When the generator starts there will be a small break in EPS power (~50 milli seconds) and then the inverter will run on the generator supply - if you don't want the inverter to try and back feed set your export limit to 0.
The inverter will see everything the generator produces as external generation (as if it were another solar inverter) - any excess above the house load will be used to charge the batteries automatically no settings required as long as the generators regulation is good enough - when the batteries are full the inverter won’t attempt to use the batteries as long as the generator output covers the house load.
Unless you are running totally off-grid i.e. no connection to the any grid - in which case you will be running in EPS mode - the problem then is that anything that you connect to the grid input will force the inverter to switch back from EPS cutting off your off-grid supplies.
In which case you would need to set the EPS circuit so that it is live when the grid is active i.e. you may need to change the setting on bypass relay and turn UPS on - then connect the generator to the grid input.
When the generator starts there will be a small break in EPS power (~50 milli seconds) and then the inverter will run on the generator supply - if you don't want the inverter to try and back feed set your export limit to 0.
Thanks Dave for your advice. I have wired up the generator input line on the opposite side to the battery shed consumer unit (which is there to feed mains AC to two the tracker controllers). The working set up today is in EPS off grid with no mains tie in at all and no ability (no cable feed) for mains tie in, so if I understand correctly:
Firing up the generator on the grid input for the inverter would override the solar input. This makes sense as the inverter has power limitations so I can see why that would be a safety feature of this input or that input. If I were to only use generator feed battery top off after dark then I wouldn't be wasting solar and the PV1 plus PV2 input would be zero. I assume once the generator power was switched off then things would revert back to seeking EPS, ready for when the new day dawns? Is that a correct assumption? I know if I can get the batteries to 80%+ charge status after dusk then there will be enough to see through the night, even those long cold Winter nights when there will be a good amount of added heating load.
I'll set the 'on grid' feed in power to zero to ensure there is no possibility of back feeding. Using an 8Kwh generator on 20Kw battery storage there should never be a need to run the generator at night for more than a hour and a half. It should all work, so long as the EPS does default back into action automatically as the system set 'self use' work mode come the next day.
I have retained a grid mains switch in the house which I could just flip during the daytime and allow all the solar output to then top off the batteries but I want to run a test to see if we can survive on solar with generator backup at a cost advantage over the easier, from the comfort of the home flip a switch, solar with grid mains as the backup. If we can, then considering going fully off-grid would be a viable option. It's a big decision in the UK to move from a grid supply to go fully off grid as there is a hefty disconnection charge to add into the cost benefit analysis of the figures.
Firing up the generator on the grid input for the inverter would override the solar input. This makes sense as the inverter has power limitations so I can see why that would be a safety feature of this input or that input. If I were to only use generator feed battery top off after dark then I wouldn't be wasting solar and the PV1 plus PV2 input would be zero. I assume once the generator power was switched off then things would revert back to seeking EPS, ready for when the new day dawns? Is that a correct assumption? I know if I can get the batteries to 80%+ charge status after dusk then there will be enough to see through the night, even those long cold Winter nights when there will be a good amount of added heating load.
I'll set the 'on grid' feed in power to zero to ensure there is no possibility of back feeding. Using an 8Kwh generator on 20Kw battery storage there should never be a need to run the generator at night for more than a hour and a half. It should all work, so long as the EPS does default back into action automatically as the system set 'self use' work mode come the next day.
I have retained a grid mains switch in the house which I could just flip during the daytime and allow all the solar output to then top off the batteries but I want to run a test to see if we can survive on solar with generator backup at a cost advantage over the easier, from the comfort of the home flip a switch, solar with grid mains as the backup. If we can, then considering going fully off-grid would be a viable option. It's a big decision in the UK to move from a grid supply to go fully off grid as there is a hefty disconnection charge to add into the cost benefit analysis of the figures.
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Dave Foster
- Posts: 1558
- Joined: Thu Oct 13, 2022 7:21 pm
Yes once the inverter detects loss of grid voltage it reverts back to EPS only modebaz0000 wrote: Tue Aug 06, 2024 10:04 am I assume once the generator power was switched off then things would revert back to seeking EPS, ready for when the new day dawns? Is that a correct assumption?
That is reassuring to be told it'll revert automatically to seeking out the solar input when the mains voltage is zero.
I just need to build a somewhat secure generator enclosure as that is an expensive piece of kit to have sitting outside unsecured. I've looked online for suitable options and they are silly money compared to a DIY effort. Back to the drawing board I'll head.
I just need to build a somewhat secure generator enclosure as that is an expensive piece of kit to have sitting outside unsecured. I've looked online for suitable options and they are silly money compared to a DIY effort. Back to the drawing board I'll head.
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Andrew37423
- Posts: 2
- Joined: Tue Jul 25, 2023 12:01 am
Hi baz, did you have any success charging the batteries from your generator, thanks.
No success.
Every hurdle jumped presented a fresh hurdle to get over.
The last hurdle that I gave up on called a failed earthing on the incoming AC feed. Maybe if I tied the generator to an earthing rod this hurdle could have been navigated but there was no guarantee it wouldn't then just be onto the next hurdle. So, I gave up.
Every hurdle jumped presented a fresh hurdle to get over.
The last hurdle that I gave up on called a failed earthing on the incoming AC feed. Maybe if I tied the generator to an earthing rod this hurdle could have been navigated but there was no guarantee it wouldn't then just be onto the next hurdle. So, I gave up.
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Andrew37423
- Posts: 2
- Joined: Tue Jul 25, 2023 12:01 am
Hi baz,
I too have fought on trying to get the batteries charged, and met with the same earthing issue witch I did manage to get past, but could never get it continuously charge, as the load increased it pulled the generator (5kw) down and seems to fall below required specs.
I'm still playing about with settings and generator speed when I get the opportunity but still jumping as you put it. Do you have your inverter grounded with an earth rod?
I too have fought on trying to get the batteries charged, and met with the same earthing issue witch I did manage to get past, but could never get it continuously charge, as the load increased it pulled the generator (5kw) down and seems to fall below required specs.
I'm still playing about with settings and generator speed when I get the opportunity but still jumping as you put it. Do you have your inverter grounded with an earth rod?
Did you get around the earth issue by hooking the generator to a good earth or by deselecting the check earth box in the agent level inverter setup? In the agent level you can also stretch out the voltage and current range if you find your generator is struggling to maintain either under load.
Seeing Fox can monitor the system remotely I figured if you start manipulating these settings then it may well void any warranty claims.
Seeing Fox can monitor the system remotely I figured if you start manipulating these settings then it may well void any warranty claims.
Hi all,
I can confirm this does work, I have it working successfully and stably and will try and remember all details.
Firstly, this thread was deeply useful for supporting my experiments. I live in a house with no grid (but gas and water) and have been running from a Fox KH8 and 10kwh of batteries and 25 panels.
For a while I made do with this and I’d miscommunicated with my installer on what I wanted and they installed an MTS (manual transfer switch) – this saw me though a few dim months as I could switch to a generator directly running the whole house while the batteries had chance to charge. I could also charge a 2.4kWh PowerOak to run the house at night. The generator I was using is a Champion Mighty Atom. I ran it from 19kg propane tanks. It would use about 500g / hour generating 500W (no really, and about 1kg / hour running 1000W)
Alas, this didn’t cut it so I got the installers back and asked them to install a 32A commando next to the generator (but not decom the MTS, which I love) connected to the GRID connector. Note that my house is connected to the EPS connector. I also upgraded my batteries to a “full stack” of 7 / 32kWh. More about this later. They did this but we could not get the generator to power the inverter as it rejected the floating earth on the Champion Mighty Atom. As a solution I created a bonded socket. Given the Mighty Atom is fully enclosed by a plastic cover I was ok with the dangers this can create. An ex Royal Signal told me how to do this so make your own mind up. Hey preso, it worked instantly. Note that you also have to set the batteries to forced charge in the Fox app from 00:01 to 23:59. There are threads on doing this so I won’t go into detail here. Note also, I was doing all of this in December after 16:00 so there was no sun on the panels. In this setup, the inverter hands off completely to the generator and doesn’t draw from the batteries, but WILL draw from the solar. Not sure why exactly, but provision your generator correctly.
Alas, something very strange happened. The inverter slowly ramped up the “mighty” atom until it tripped at a charge rate of about 2200W. It did this three times and eventually stopped drawing extra current at about 2150W but the poor mighty atom was NOT happy. It’s as if the inverter can learn the max supply current it can take, quite smart I guess.
Eventually I found a setting on the inverter which limits the input current. It’s under settings -> Battery -> Max Chg. Cur. You need to know the voltage of your batteries and set the current to something else. I also bought a larger Challenge LPG generator that was rated at 3000W and had an electric starter. I set the current at 3-5 amps, which on a full stack is about 330V so the battery charges at 1000W to 1500W. I do have neighbours-ish so I run it from 16:00 to 20:00 and so I get 4-6kWh of charge into the batteries. The major downside is you HAVE to remember to turn the current back up after or you risk throttling your solar generator. On some of the forums there is apparently a setting you can see via the API / Modbud called “max current on grid” – but I didn’t want to start down that path given my installers would have kittens.
One thing I don’t like is when I turn on the generator and flip with 32A switch you can hear several clicks from the inverter. I don’t know if they are solenoids etc but it’s quite a loud “click click click” as it is negotiating with the input. It’s probably not good for the inverter in the long run. When you turn off the generator I’d say the gap is around 0.5 – 1 second. Everything that doesn’t have a decent survival rate will reset, for example the cooker clock resets, the TV turns off, etc. So if you have, say, a desktop PC and no UPS, yes it will likely turn off. It’s not 50ms for the EPS to come back on, definitely feels a lot longer.
I have to say, as well as this all works, if I get a sunny winter day, with a 20+ panels you can get 10-12kWh (I’m in the SE) so I’ve actually barely needed to use the generator, maybe 30 hours run time all through Dec and Jan… Upgrading the battery tower was absolutely a good move. I’ve set the min Soc to 10% but I’ve not been below 40% for a while.
I had planned to connect the generator to the mains gas, hence the LPG ones, but I just haven’t found the need. To refill a 19kg Calor propane is about £65 and that will generate about 15-20kWh from the generator I have.
It’s all been a bit stressful and it’s looking like I can get a mains connection sometime in 2025. I could probably live forever without it by my wife is a hard NO on that haha.
I hope this helps the next person plan. I was pleasantly surprised that the fox kit could even do any of this. I did a lot of work looking at Victron but 48V was just not enough.
If anyone wants me to write more about this or I’ve missed anything let me know. My installers are gold also so happy to share their details directly with people.
I can confirm this does work, I have it working successfully and stably and will try and remember all details.
Firstly, this thread was deeply useful for supporting my experiments. I live in a house with no grid (but gas and water) and have been running from a Fox KH8 and 10kwh of batteries and 25 panels.
For a while I made do with this and I’d miscommunicated with my installer on what I wanted and they installed an MTS (manual transfer switch) – this saw me though a few dim months as I could switch to a generator directly running the whole house while the batteries had chance to charge. I could also charge a 2.4kWh PowerOak to run the house at night. The generator I was using is a Champion Mighty Atom. I ran it from 19kg propane tanks. It would use about 500g / hour generating 500W (no really, and about 1kg / hour running 1000W)
Alas, this didn’t cut it so I got the installers back and asked them to install a 32A commando next to the generator (but not decom the MTS, which I love) connected to the GRID connector. Note that my house is connected to the EPS connector. I also upgraded my batteries to a “full stack” of 7 / 32kWh. More about this later. They did this but we could not get the generator to power the inverter as it rejected the floating earth on the Champion Mighty Atom. As a solution I created a bonded socket. Given the Mighty Atom is fully enclosed by a plastic cover I was ok with the dangers this can create. An ex Royal Signal told me how to do this so make your own mind up. Hey preso, it worked instantly. Note that you also have to set the batteries to forced charge in the Fox app from 00:01 to 23:59. There are threads on doing this so I won’t go into detail here. Note also, I was doing all of this in December after 16:00 so there was no sun on the panels. In this setup, the inverter hands off completely to the generator and doesn’t draw from the batteries, but WILL draw from the solar. Not sure why exactly, but provision your generator correctly.
Alas, something very strange happened. The inverter slowly ramped up the “mighty” atom until it tripped at a charge rate of about 2200W. It did this three times and eventually stopped drawing extra current at about 2150W but the poor mighty atom was NOT happy. It’s as if the inverter can learn the max supply current it can take, quite smart I guess.
Eventually I found a setting on the inverter which limits the input current. It’s under settings -> Battery -> Max Chg. Cur. You need to know the voltage of your batteries and set the current to something else. I also bought a larger Challenge LPG generator that was rated at 3000W and had an electric starter. I set the current at 3-5 amps, which on a full stack is about 330V so the battery charges at 1000W to 1500W. I do have neighbours-ish so I run it from 16:00 to 20:00 and so I get 4-6kWh of charge into the batteries. The major downside is you HAVE to remember to turn the current back up after or you risk throttling your solar generator. On some of the forums there is apparently a setting you can see via the API / Modbud called “max current on grid” – but I didn’t want to start down that path given my installers would have kittens.
One thing I don’t like is when I turn on the generator and flip with 32A switch you can hear several clicks from the inverter. I don’t know if they are solenoids etc but it’s quite a loud “click click click” as it is negotiating with the input. It’s probably not good for the inverter in the long run. When you turn off the generator I’d say the gap is around 0.5 – 1 second. Everything that doesn’t have a decent survival rate will reset, for example the cooker clock resets, the TV turns off, etc. So if you have, say, a desktop PC and no UPS, yes it will likely turn off. It’s not 50ms for the EPS to come back on, definitely feels a lot longer.
I have to say, as well as this all works, if I get a sunny winter day, with a 20+ panels you can get 10-12kWh (I’m in the SE) so I’ve actually barely needed to use the generator, maybe 30 hours run time all through Dec and Jan… Upgrading the battery tower was absolutely a good move. I’ve set the min Soc to 10% but I’ve not been below 40% for a while.
I had planned to connect the generator to the mains gas, hence the LPG ones, but I just haven’t found the need. To refill a 19kg Calor propane is about £65 and that will generate about 15-20kWh from the generator I have.
It’s all been a bit stressful and it’s looking like I can get a mains connection sometime in 2025. I could probably live forever without it by my wife is a hard NO on that haha.
I hope this helps the next person plan. I was pleasantly surprised that the fox kit could even do any of this. I did a lot of work looking at Victron but 48V was just not enough.
If anyone wants me to write more about this or I’ve missed anything let me know. My installers are gold also so happy to share their details directly with people.
Last edited by Max Power on Sat Jan 25, 2025 11:25 pm, edited 1 time in total.