Rewiring existing circuits to UPS output of solar/battery inverter

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Feb 14, 2023
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We have a Alpha ESS hybrid inverter+10kWh battery with a UPS output. Having had a lengthy power cut recently (caused by cable theft) I want to make use of the UPS facility. The inverter is rated at 5kW. It has double pole 32A breakers on both the mains connection and the UPS output.

My aim is that in the event of a power cut, I will be able to run the freezer, some of the lighting and the heating from the UPS output, and have WiFi internet access from mobile devices.

So far I have run a 2.5mm^2 flex cable (similar to the inverter mains input cable installed by original electrician) from the UPS output to a metal-cased 5-position consumer unit with pre-fitted 40A RCB (hereafter called the UPS CU). From that I have run a short 2.5mm^2 T+E cable to a double socket, protected by a 20A MCB in the CU. A mains tester (the old sort with neon indicators) plugged into the socket shows normally (3 lights) when mains power is supplied. If I turn off the incoming mains feed to the inverter, it goes into UPS mode. The mains tester then reads "no earth". This appears to be because in UPS mode the inverter produces a balanced output - I measure 120V between line and earth, 120V between neutral and earth, 240V between line and neutral.

What I would like to do, with the help of an electrician at least to do a final inspection and tests, is:
1. Add a radial circuit to the same 20A MCB in the UPS CU (so it will become a branching radial circuit) to two power sockets in an adjoining passage that power the freezer and WiFi router, using 2.5mm^2 T+E. These sockets are currently spurred off existing ring circuits.
2. Possibly remove the kitchen fridge/freezer socket from the existing kitchen ring circuit (adding junction boxes as necessary to preserve the ring) and connect it to the same radial circuit
3. Currently the oil-fired central heating control system is powered from the immersion heater circuit through a 3A fused spur. I propose to separate it and power it instead from the UPS CU via a 6A MCB.
4. I would also like to connect one of the two existing lighting circuits to another 6A breaker in the UPS CU.

Can anyone see a practical or legal problem with this arrangement? I have read that neutrals from different circuits occasionally get connected together other than at the CU, so I would check that the circuits I re-route to the UPS CU don't suffer from this.

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