Off-grid generator integration Sizing, auto-start, and charging done right
A generator is the backbone of a reliable off-grid system — but only if it's correctly sized, properly integrated, and sensibly configured. This guide covers everything from generator kVA selection through auto-start SoC thresholds, charge current limits, and runtime efficiency.
We remotely diagnose generator auto-start faults, overload trips, charge current issues, and runtime problems — and configure the correct settings for your inverter-charger and generator combination.
Book a consultation — £75 → All guidesSize the generator correctly for your inverter-charger
The most common mistake in off-grid installs is pairing an undersized generator with a full-power inverter-charger — the generator overloads and cuts out.
These figures include headroom for AC loads running during charging. Always use the generator's continuous rated output — not the peak or maximum figure printed on the label. A 7kVA generator with a 5.5kW continuous rating is only a 5.5kW generator for practical purposes.
Configure auto-start SoC and voltage thresholds
Correct auto-start thresholds prevent both battery over-discharge and unnecessary generator runtime.
Start at 20–30% SoC, stop at 80–90% SoC. On Victron Cerbo GX: Settings → Generator start/stop → SoC condition. The stop SoC should be high enough to complete a meaningful charge but not so high that the generator runs for hours when solar can finish the job.
Set 30–60 minutes. This prevents the generator starting, bringing the battery above the stop SoC quickly, and shutting down — then repeating on the next SoC dip. Short-cycling wears the engine starter motor and prevents the battery from completing a full charge cycle. On Victron: Settings → Generator start/stop → Minimum run time.
Set a backup voltage start condition (e.g. 46.0V on a 48V LiFePO4 system) for cases where the SoC estimate drifts. The voltage trigger acts as a safety net regardless of displayed SoC.
Raise the start SoC to 35–40% in winter to account for cold-weather capacity loss and reduced solar generation. See our cold weather battery performance guide for the full picture on winter system settings.
On Victron Cerbo GX, you can add quiet-time conditions to block auto-start between set hours (e.g. 22:00–07:00) so the generator never starts at night automatically. Manual override remains available.
Set the correct AC input current limit and charge profile
The AC input limit protects the generator from overload. The charge profile determines how the battery charges — and both must be set correctly.
The 0.8 factor provides headroom for load transients and power factor variation. On Victron MultiPlus-II: set AC input current limit in VEConfigure → Grid tab, or remotely via VRM. On Cerbo GX: Settings → ESS → AC input current limit.
Balance generator runtime and maximise fuel efficiency
Every generator hour costs money. Running it strategically significantly reduces annual fuel spend.
When the generator runs, use it to power washing machine, dishwasher, kettle, power tools, or immersion heater simultaneously. This maximises the useful energy output per litre of fuel. Running only the battery charger at 50% of generator capacity wastes the other 50% entirely.
In UK summers, many off-grid systems rarely need the generator. Run it under meaningful load for 30–60 minutes every two to four weeks: this keeps the alternator charged, circulates fresh oil through the engine, prevents fuel gumming in the carburettor (petrol), and ensures it will start reliably when needed. Use a fuel stabiliser in any petrol stored for more than four weeks.
Installation, transfer switching, and maintenance planning
A generator that isn't properly installed or maintained will fail when you need it most.
The Victron MultiPlus-II contains a built-in transfer relay — when the generator AC output is connected to the AC-in terminal and the generator is running, the inverter transfers loads automatically. There is no need for an external ATS. For systems where the generator feeds an external distribution board, an external manual or automatic transfer switch is required to prevent backfeed to the inverter AC output.
Never operate a generator indoors, in a garage, or in any enclosed or semi-enclosed space. Carbon monoxide is odourless and lethal. Fixed generator enclosures must be vented with sufficient air flow for combustion and cooling. Install CO detectors in any adjacent habitable spaces. Site the exhaust to discharge away from windows, doors, and air intakes. For a generator in a purpose-built acoustic housing, ensure the enclosure is MCS or manufacturer-approved for the generator model.
Petrol and diesel are Class II and Class IIIA flammable liquids under DSEAR regulations. Storage in excess of 30 litres outside a dwelling requires consideration of DSEAR risk assessments. Fixed generator enclosures or outbuildings housing generators may require planning permission if they exceed permitted development limits — check with your local planning authority before constructing a permanent installation.
Petrol generators: engine oil every 50–100 hours or annually, air filter every 100 hours, spark plug every 200 hours. Diesel generators: engine oil every 100–200 hours, fuel filter annually, coolant every two years (liquid-cooled models). Keep a written service log — it matters for insurance and warranty claims.
Generator integration — common questions
The most common causes are: the minimum run time setting is too short (set at least 30–60 minutes), the auto-stop SoC is set too low so the battery reaches it quickly, or the generator is overloading and shutting down because the AC input current limit exceeds the generator's capacity. Check the Cerbo GX event log for the stop reason. If it shows an overload or AC input fault, reduce the AC input current limit in VEConfigure. If it stops cleanly at SoC, raise the stop SoC to 85–90% and check minimum run time.
Size so the generator can supply the inverter-charger's maximum AC demand plus any simultaneous loads. For a MultiPlus-II 5000VA at 100A charge current into a 48V battery: approximately 4.8kW charger draw plus background loads means you need a 6–7kW continuous-rated generator. A practical rule: generator kW ≥ inverter kVA × 0.9. Always use the continuous rating, not the peak or maximum figure. Inverter generators are best for smaller systems or quiet-running requirements; diesel generators are preferred for larger off-grid systems needing regular reliable charging.
On Cerbo GX: Settings → Relay → Relay 1 → set function to Generator start/stop. Then Settings → Generator start/stop → configure conditions. Set start SoC (e.g. 25%), stop SoC (e.g. 85%), minimum runtime (30–60 minutes), and optionally a voltage backup trigger (e.g. 46V on a 48V system). Add quiet-time conditions if you want to block auto-start between set hours. On non-Victron inverter-chargers, the generator start relay is typically a dry-contact output — consult your inverter manual for the configuration menu path, as the settings are equivalent but accessed differently.
Yes — on most off-grid systems, MPPT solar charging and generator AC charging run simultaneously. On Victron systems with DVCC, the BMS sends a total charge current limit (CCL) that both the MPPT controllers and the MultiPlus charger share. Solar fills its available current and the MultiPlus makes up the difference up to the CCL. This is the correct and most efficient configuration. Never parallel generator AC output directly with inverter AC output without a proper transfer switch or an AC coupling configuration designed for it — unsynchronised AC sources must not be directly connected.
The most common causes: the AC input current limit is set lower than expected (check VEConfigure → Grid tab); the BMS charge current limit (CCL) is capping the combined input from solar and the generator charger; the battery is near full SoC and the charger is naturally tapering; or the generator is an inverter type that reduces output at part load — this is normal. Check charge current in real time via Victron VRM or the Cerbo GX display. If the BMS CCL is the limiting factor, check the battery's temperature and SoC — a cold battery significantly reduces the CCL, and this is the correct protective behaviour.
Generator not integrating correctly?
Tell us your generator model, inverter-charger, and the problem you're seeing. We'll identify the configuration issue — auto-start settings, charge current limits, or overload fault — and provide the correct settings for your exact setup.
Request received
We'll review your generator and inverter details and respond within one working day.