A hybrid inverter connects to the grid, a separate backup panel, a battery and solar panels. Below: how these parts fit together, how to protect each line, how to size the battery cable and in what order to power the system up the first time. This explains the principle — it is not a design: work inside the distribution board belongs to an electrician.
Safety: direct current from a 48 V battery means hundreds of amps. A short across the terminals creates an arc that doesn't extinguish itself the way an AC arc does, and it melts tools. Do all work with breakers off and the battery isolator open.
What the system consists of
- The grid supply with its main breaker — as before.
- The inverter, which receives the grid and feeds the backup circuits.
- A backup panel — a separate group of breakers for appliances that must work during an outage.
- The battery with its own protection and a BMS communication cable.
- Solar panels (if any) with a DC isolator.
- A current sensor or meter at the supply — to control export, if the model supports it.
Hybrid inverter ports
| Port | What connects | Note |
|---|---|---|
| GRID | the incoming supply | through a dedicated breaker |
| LOAD / BACKUP | the backup panel | powered by both grid and battery |
| GEN / SMART LOAD | a generator or non-essential load | model-dependent |
| BAT | the battery | through a DC breaker or fuse |
| PV | solar panels | through a DC isolator |
| CT / METER | current sensor at the supply | to limit export to the grid |
| BMS | battery communication cable | CAN or RS485 |
Port names vary between manufacturers, but the logic is the same: the grid goes into the inverter, and the backup circuits come out of it.
Which circuits to move to backup
Move onto the LOAD output what you can't do without during an outage:
- fridge and freezer;
- gas boiler and circulation pumps;
- lighting (one or two circuits);
- router, cameras, alarm;
- sockets for laptops and chargers;
- the well pump — if the inverter can handle its start-up.
Leave heavy loads — electric stove, water heater, air conditioning — on the grid, or connect them to the GEN/SMART LOAD output, which the inverter switches on only when there's surplus energy. Otherwise someone at home switches on the kettle and water heater together and either drains the battery in an hour or overloads the inverter.
Battery cable and protection
The inverter-to-battery cable should be as short as possible — 2–3 metres at most — with a flexible stranded copper core. Approximate cross-sections for 48 V systems:
| Inverter | Battery current (approx.) | Copper cross-section | Protection rating |
|---|---|---|---|
| 3 kW | up to 70 A | 16–25 mm² | 100 A |
| 5 kW | up to 120 A | 35 mm² | 150–160 A |
| 8 kW | up to 190 A | 50 mm² | 250 A |
| 12 kW (3-phase) | up to 240 A | 70 mm² or 2 × 35 mm² | 300 A |
This is a guide for short runs in free air. Take the final cross-section and protection rating from the inverter manual, which states the maximum battery current for the specific model.
- Place the protection on the positive cable as close to the battery as possible: a breaker or fuse rated for DC and the battery voltage. An ordinary AC breaker cannot break a DC arc.
- Lugs must be crimped, sized for the bolt, and tightened to the torque in the manual. A loose terminal at 200 A heats up until it melts.
- Always connect the BMS communication cable: through it the battery tells the inverter the permitted currents and charge voltage. Settings are covered in LiFePO4 charge settings.
Solar panels: voltage in the cold
A panel's open-circuit voltage (Voc) rises in the cold — by roughly 0.25–0.3 % for every degree below 25 °C. A string that fits the inverter's limits in summer can exceed them on a February morning.
Example: a panel with a Voc of 49.5 V and a coefficient of −0.28 %/°C gives about 55.7 V at −20 °C. Ten of them in series make 557 V — above the 500 V maximum of many inverters. That string has to be cut to nine panels.
- Fit a DC isolator between the panels and the inverter so the line can be de-energised for service.
- Check polarity with a multimeter before connecting: a reversed string can destroy the input.
Earthing and RCDs
- Bond the inverter case, battery rack and panel frames to the building's protective earth (PE).
- Fit RCDs or RCBOs on the backup panel outputs, as on ordinary circuits.
- In backup mode the inverter forms the neutral itself. Check in the manual how your model links neutral and earth in off-grid mode — this determines whether the RCD will trip correctly when the grid is down.
Connection and first start-up order
- Mount the inverter and battery, and bond all enclosures to PE.
- Connect the battery with the DC breaker open, and the BMS communication cable.
- Connect the grid to GRID and the backup panel to LOAD — all breakers off.
- Connect the panels with the DC isolator off, after checking string polarity and voltage.
- Switch on the battery, then the DC breaker — the inverter should start from the battery.
- Configure the battery type, BMS protocol, currents and source priorities.
- Apply the grid, then switch on the panels.
- Only then switch on the backup circuit breakers one at a time, watching the load.
Common mistakes
- The whole house on the LOAD output — the first water heater overloads the inverter.
- An AC breaker in the battery circuit instead of a DC breaker.
- A thin or long battery cable — losses, heat, and early cut-outs under load.
- BMS communication not connected — the inverter charges "blind" by voltage.
- A solar string sized without allowing for frost.
Summary
The grid goes into the inverter, essential circuits come out of it, heavy loads stay on the grid. The battery connects with a short, thick cable through DC protection; the panels through a DC isolator with voltage headroom for frost. Before installation, settle on the inverter and battery capacity, and leave the work to an electrician.