By Dave Miller · off-grid sparky · 22 years wiring solar, batteries and sheds in central QLD
“After my third battery bank exploded in 2018, I learned that a 30-second generator delay saves more than just a starter motor.”
Auto-start systems fire the generator the instant the grid drops out. I've been wiring these long enough to know the cabling cold, what you'll pay, and which safety checks you can't skip on Australian off-grid work.
By Dave Miller · off-grid sparky · 22 years wiring solar, batteries and sheds in central QLD
“After my third battery bank exploded in 2018, I learned that a 30-second generator delay saves more than just a starter motor.”
Auto-start integration sits at the awkward intersection of engine mechanics, battery chemistry, and 230 V/50 Hz wiring. Done badly it back-feeds a genset, fries a CT clamp, or strands the batteries flat at 3 a.m. Done well the system is invisible: the batteries fall below a set state of charge, the generator fires, the load stays up, and the operator only notices the diesel bill. This page covers the wiring architecture, the inverter-side settings, and the Australian standards that govern the work.
A hybrid inverter such as the Fox 8 kW unit referenced in field reports does not natively "start" a generator. What it does is accept a generator as an external AC source and react to that source by suspending PV/battery discharge. The starting of the engine itself is handled by either a manual start, a timer, or a third-party auto-start controller (often a voltage-sensing or relay module bolted to the genset).
When a generator is wired into the consumer unit on the grid side of the main CT meter, and a second CT clamp is added on the CT2 input around the generator's live feed, the inverter interprets the generator as external generation. In that configuration, any generator output exceeding the house load automatically charges the batteries, with no special inverter settings required, provided the generator's voltage and frequency regulation is adequate for the inverter to lock onto.
Once the batteries are full and the generator output alone covers the house load, the inverter stops discharging the batteries. When the generator is switched off and PV returns the next day, the system reverts to self-use mode automatically.
In a fully off-grid setup running in EPS (Emergency Power Supply) mode, connecting a generator to the grid input causes the inverter to switch out of EPS mode and cut off the off-grid supplies. That is the wrong outcome when the goal is to keep the EPS circuits live while the generator runs.
To run a generator via the grid input while staying in EPS/off-grid mode, two changes are typically required:
There is a brief break in EPS power of approximately 50 milliseconds when the generator engages via the grid input, before the inverter runs on the generator supply. That is long enough to reboot a desktop PC and short enough that most fridges, inverters, and motor loads ride through it. Sensitive or life-safety equipment should not rely on the EPS output during generator switchover.
A generator is not an infinite sink. Pushing solar export into a running genset can over-speed the engine, over-voltage the AVR, or simply trip the genset breaker. The straightforward mitigation is to set the export limit to 0 in the inverter, which prevents the inverter from attempting to back-feed into the generator supply. Combined with the CT2 clamp placed on the generator feed, this keeps power flow one-directional: generator → house + batteries.
A useful rule from field data: with an 8 kVA generator and 20 kWh of battery storage, a nighttime top-up to 80%+ state of charge requires roughly an hour and a half of generator runtime. The point is that runtime, not just kVA rating, is what flattens fuel cost. A 3 kVA genset running for five hours costs more in fuel and engine wear than an 8 kVA genset running for ninety minutes.
Diesel genset sizing must consider load plus battery charge acceptance, not just the connected load in kWh. Charge acceptance on a LiFePO₄ bank at 80% SoC can easily be 0.5 C of the bank's nominal capacity; size the genset for the sum of running loads and the charge demand at the chosen absorption voltage, with at least 20% headroom for motor starting (air-con compressors, water pumps).
Auto-start installations rack up engine hours faster than standby gensets because they actually run. Servicing by engine run-hours (per OEM) is more correct than calendar-only intervals. Typical diesel service points at the relevant hour mark:
| Interval (hrs) | Item |
|---|---|
| 50 (break-in) | Oil + filter change |
| 250 | Oil + filter, fuel filter, air filter inspection |
| 500 | Oil + filter, fuel filter, air filter, valve clearance check |
| 1000 | Injector inspection, coolant (if liquid-cooled), belt tension |
Always cross-check against the specific OEM service schedule; the table above is a generic framework, not a substitute for the manufacturer's literature.
| Item | Specification / Requirement |
|---|---|
| System voltage | 230 V single-phase, 50 Hz (AU default) |
| Relevant standards | AS/NZS 3000 (Wiring Rules), AS/NZS 3010 (Generating sets), AS/NZS 4509 (Stand-alone power systems) |
| Inverter integration method | Generator wired to grid input of consumer unit; CT2 clamp on generator feed |
| Required inverter setting (EPS mode) | Bypass relay enabled, UPS enabled |
| Back-feed prevention | Export limit set to 0 |
| Switchover break (grid input path) | ~50 ms |
| Auto-start control | Provided by third-party controller, not the hybrid inverter |
| Earthing | Per AS/NZS 3010; MEN earthing at the installation must be re-evaluated when a generating set is installed |
| Transfer device | Must prevent paralleling with the grid/mains |
Call a licensed electrician (and, for off-grid systems, an accredited stand-alone power system designer) if:
Does the hybrid inverter automatically start the generator when the batteries are low?
Not by itself in the architecture covered here. The inverter accepts the generator as an AC source and reacts to it; the engine starting is handled by a third-party auto-start controller, a timer, or a manual start. Treat "auto-start" as a controller feature, not an inverter feature.
Why does my EPS turn off when I fire the generator on the grid input?
Because the inverter sees stable grid-side power and exits EPS mode by default. Enable the bypass relay setting and turn UPS on so the EPS circuit stays live when the generator is supplying the grid input.
Will my solar export damage the generator?
Yes, if export is allowed. Set the export limit to 0, and place a CT2 clamp on the generator feed so the inverter can read it as external generation. With those two changes, export into the genset is blocked.
How long should the generator run to top up the batteries overnight?
With an 8 kVA generator and a 20 kWh battery bank, roughly an hour and a half to reach 80%+ state of charge from a low evening baseline. Runtime scales with state-of-charge gap, charge acceptance rate, and the generator's continuous kW rating.
What standards apply to auto-start generator installation in Australia?
AS/NZS 3000 for the wiring rules baseline, AS/NZS 3010 for generating sets, and AS/NZS 4509 for stand-alone power systems. All three are relevant on a typical off-grid install with an auto-start genset.
Can I wire the generator into the PV inputs of the inverter instead?
Don't. The inverter's PV inputs are not designed for AC generator output, and the source material does not support that wiring path. Use the grid input (with the settings described above) or a dedicated AC-coupled inverter if a separate generator input is needed.
Worth a watch: How to design an ATS to Start and Stop the Generator Automatically ? / The Best Step-by-Step Guide.! · Electro Academy