A minimal EPS setup with an H3 inverter
Posted: Sat Nov 07, 2026 11:33 am
In case this is of any use to anyone with a similar setup/needs: this is how I've set up my system to provide some minimal backup if/when the grid supply goes down.
Background: I want to be able to run my oil-fired boiler, circulation pumps and a few small devices (e.g. chargers etc) when the grid is out - a peak load of about 500W when the boiler is firing, and about 100W when just circulating. I don't need this to auto-switch or provide UPS-style backup, and I'm happy to have to do a bit of setup if and when the need arises.
I'm not a prepper as such, and the supply here is generally very reliable - but it's only a matter of time before the Russians get around to disrupting the grid, and they won't do that in the summer when the sun is shining. We can cook using a stove and some of the 5000L of heating diesel sitting in tanks in the cellar, and lighting's no problem with rechargeable lamps - but keeping the house warm at -10C would be a much bigger problem.
Kit: I've got 7.5kWh of panels, an H3 Smart 10.0 inverter, and a 7.5kWh EQ battery. It's wired to the house supply via a 40A 4-pole RCD and a 32A MCB in series. The solar system has its own grounding rod, but the solar and house earths are tied together. All of this was professionally installed to what I think was a very high standard, and the house systems were completely reinstalled two years ago.
Briefly, the solution is this:
- a 4mm five-core cable between the inverter's AC connector and the bus bar inside a junction box on the wall next to the inverter
- a free-standing distribution box with 2x16A Schuko sockets and a DIN rail
- a length of flexible 4mm five-core cable with the loose end pre-crimped ready to be connected to the junction box buses.
- on the DIN rail in the distribution box are two 16A RCBOs connected to L1 and L2, and a 16A switch which bonds the box's N to the PE when closed.
- I've replaced the hardwired connection of the boiler with a surface mount socket on the house circuit, and a standard Schuko plug on the line to the boiler.
When the grid goes out, the process will be to isolate the inverter from the house by opening the RCD and MCB, and connecting the distribution box to the inverter via the junction box. Then the boiler can be plugged into L1 in the distribution box which will allow me run the heating and water off battery, with L2 as backup for other small devices.
The H3 documentation was unclear about whether the inverter handles N-PE bonding itself - I found an undocumented menu option which suggests it's configurable, so to cover my options I've added a switchable bond in the distribution box. Part of the procedure I've written for myself is checking for the N-PE bond while connecting everything - during the test run I did, it turned out that the inverter does do it internally, but I figured that adding the flexibility was easy enough. It also means I'm covered in case it's software-controlled and gets flipped during a firmware update or whatever.
Legally I'm in a grey area here. I'm not formally electrically-qualified, but I am an electrical engineer by background - I'm happy that I'm not going to kill myself or burn the place down, but it is modifying a fixed installation (at least to the extent of adding an additional unconnected link to the AC connector). On the other hand, using it in practice is a temporary fix, and if the grid is out to the extent that I need it, then sticking to the letter of the law is going to be the least of my concerns...
Background: I want to be able to run my oil-fired boiler, circulation pumps and a few small devices (e.g. chargers etc) when the grid is out - a peak load of about 500W when the boiler is firing, and about 100W when just circulating. I don't need this to auto-switch or provide UPS-style backup, and I'm happy to have to do a bit of setup if and when the need arises.
I'm not a prepper as such, and the supply here is generally very reliable - but it's only a matter of time before the Russians get around to disrupting the grid, and they won't do that in the summer when the sun is shining. We can cook using a stove and some of the 5000L of heating diesel sitting in tanks in the cellar, and lighting's no problem with rechargeable lamps - but keeping the house warm at -10C would be a much bigger problem.
Kit: I've got 7.5kWh of panels, an H3 Smart 10.0 inverter, and a 7.5kWh EQ battery. It's wired to the house supply via a 40A 4-pole RCD and a 32A MCB in series. The solar system has its own grounding rod, but the solar and house earths are tied together. All of this was professionally installed to what I think was a very high standard, and the house systems were completely reinstalled two years ago.
Briefly, the solution is this:
- a 4mm five-core cable between the inverter's AC connector and the bus bar inside a junction box on the wall next to the inverter
- a free-standing distribution box with 2x16A Schuko sockets and a DIN rail
- a length of flexible 4mm five-core cable with the loose end pre-crimped ready to be connected to the junction box buses.
- on the DIN rail in the distribution box are two 16A RCBOs connected to L1 and L2, and a 16A switch which bonds the box's N to the PE when closed.
- I've replaced the hardwired connection of the boiler with a surface mount socket on the house circuit, and a standard Schuko plug on the line to the boiler.
When the grid goes out, the process will be to isolate the inverter from the house by opening the RCD and MCB, and connecting the distribution box to the inverter via the junction box. Then the boiler can be plugged into L1 in the distribution box which will allow me run the heating and water off battery, with L2 as backup for other small devices.
The H3 documentation was unclear about whether the inverter handles N-PE bonding itself - I found an undocumented menu option which suggests it's configurable, so to cover my options I've added a switchable bond in the distribution box. Part of the procedure I've written for myself is checking for the N-PE bond while connecting everything - during the test run I did, it turned out that the inverter does do it internally, but I figured that adding the flexibility was easy enough. It also means I'm covered in case it's software-controlled and gets flipped during a firmware update or whatever.
Legally I'm in a grey area here. I'm not formally electrically-qualified, but I am an electrical engineer by background - I'm happy that I'm not going to kill myself or burn the place down, but it is modifying a fixed installation (at least to the extent of adding an additional unconnected link to the AC connector). On the other hand, using it in practice is a temporary fix, and if the grid is out to the extent that I need it, then sticking to the letter of the law is going to be the least of my concerns...