Introduction
Every evening at 19:47, the generator started automatically. Every evening at 20:19 to 20:22, it stopped. The Cerbo GX log showed the generator running for 85 to 95 seconds on each automatic start before the Multiplus-II dropped the AC input and returned to inverter mode. The battery state of charge at the end of each generator run was within 1 percent of where it had been at the start. The generator was consuming fuel, accumulating hours, and delivering nothing.
The installer who configured the system had set the AC input current limit correctly at 10A and verified the 5kVA generator’s rated output against the combined load calculation. What he had not done was measure the generator’s output voltage at the Multiplus-II’s AC input terminals under the combined load the Multiplus-II placed on it at the moment of transfer. The generator, a low-cost conventional unit with a brush AVR and a mechanical governor, produced 231V at no-load and sagged to 187V within two seconds of accepting the full combined load. The Multiplus-II’s AC input acceptance window has a lower voltage limit of 180V.
The generator was accepted at 187V, which sits just above the rejection threshold, but the Multiplus-II’s internal monitoring flagged the sustained low-voltage condition as an unstable source and dropped the input after 85 seconds to protect the battery from the degraded charge quality.
The root cause was not the AC input current limit. It was a generator with inadequate voltage regulation under load — a power quality problem that is only detectable by measuring voltage at the inverter terminals under the actual combined load. This post establishes the complete framework for generator integration: sizing, power quality verification, auto-start configuration, hybrid operating modes, and run time optimisation.
Generator Sizing for Off-Grid Solar Systems(Hybrid)
Generator sizing for a hybrid off-grid system is a three-demand calculation. The total generator demand must cover the peak AC load, the battery charging load, and a starting margin that keeps the combined demand within the generator’s safe operating envelope. The battery charging load is the most commonly omitted term in this calculation, and omitting it produces a generator that is either undersized for the combined demand or oversized for the building load alone.
P_generator_required = (P_AC_load + P_charge) / 0.80
Where:
P_AC_load = peak AC load at time of generator start (W)
P_charge = battery charge power drawn from generator (W)
0.80 = safe operating factor
The battery charge power drawn from the generator is determined by the AC input current limit configured in the Multiplus-II, not by the charger’s maximum rated current. The Multiplus-II 48/3000/35-32 has a built-in charger rated at 35A DC output, corresponding to an AC input draw of up to 8,050VA at full current. The AC input current limit is the tool that reduces this demand to a manageable level:
At AC input limit 20A:
P_total = 1,950 + (20A x 230V) = 6,550W -> P_generator = 6,550 / 0.80 = 8,188W -> 10kVA
At AC input limit 10A:
P_total = 1,950 + (10A x 230V) = 4,250W -> P_generator = 4,250 / 0.80 = 5,313W -> 6kVA
At AC input limit 8A:
P_total = 1,950 + (8A x 230V) = 3,790W -> P_generator = 3,790 / 0.80 = 4,738W -> 5kVA adequate
The 5kVA generator with the AC input current limit set to 8A produces a combined demand of 3,790W against a 5,000W rating, 75.8 percent loading, within the 80 percent safe operating envelope. The starting margin provides the transient headroom the generator needs to accept the load cleanly when the Multiplus-II transfers to passthrough mode.
| Generator | AC Input Limit | Combined Load | Loading % | Result |
| 5kVA | 8A | 3,790W | 75.8% | PASS |
| 5kVA | 10A | 4,250W | 85.0% | MARGINAL |
| 5kVA | 13A | 4,940W | 98.8% | FAIL |
| 6kVA | 10A | 4,250W | 70.8% | PASS |
For the AC input current limit configuration on the Multiplus-II, refer to our engineering guide on how to size and select an off-grid inverter — complete worked example.
AC Input Current Limit, The Critical Configuration Parameter
The AC input current limit caps the total AC current the Multiplus-II draws from its AC input source for both connected AC loads and the battery charger simultaneously. When the total AC input current reaches the configured limit, the Multiplus-II activates PowerAssist mode and begins supplementing the generator output with battery discharge current to serve any additional load demand above the limit without exceeding the generator’s capacity.
The parameter is consistently misconfigured in two ways: not setting it at all, which lets the Multiplus-II draw the full rated charger current from the generator simultaneously with the AC load; or setting it to the generator’s rated output current rather than the safe operating limit. The correct calculation:
I_AC_limit = (P_generator x 0.80 / V_AC) – I_AC_load
For cluster system with 5kVA generator:
I_AC_limit = (5,000W x 0.80 / 230V) - (1,950W / 230V) = 17.4A - 8.5A = 8.9A -> set to 8A (round down)
Setting the limit to 8A rather than 8.9A rounds down conservatively, keeping the combined demand within the safe operating envelope even when the AC load momentarily exceeds the design figure. PowerAssist mode makes the AC input current limit a soft ceiling rather than a hard cutoff. At a boost factor of 2x, a generator operating at its 8A input limit can momentarily serve a load equivalent to 16A of AC current without exceeding its rated output.
AC Input Current Limit Configuration Procedure:
Step 1 -> calculate generator safe operating current: P_generator x 0.80 / V_AC
Step 2 -> calculate peak AC load current: P_load / V_AC
Step 3 -> calculate available charging current: safe current - load current
Step 4 -> round down to nearest integer ampere
Step 5 -> set in VEConfigure under charger settings: AC input current limit
Step 6 -> verify in Cerbo GX device list: Multiplus-II shows correct limit value
Step 7 -> test under combined load: confirm generator current within safe limit
Generator Power Quality and Acceptance Criteria
The Multiplus-II’s AC input acceptance window is 180V to 265V for voltage and 45Hz to 65Hz for frequency. A generator whose output falls within these limits at the Multiplus-II’s AC input terminals under the full combined load will be accepted as a valid source. The critical qualifier is at the Multiplus-II’s AC input terminals under the full combined load. Generator output voltage varies with the load presented, the quality of the AVR, and the condition of the alternator windings.
The voltage measurement procedure requires a calibrated AC voltmeter connected at the Multiplus-II’s AC input terminals, not at the generator’s output terminals. The cable run from the generator to the inverter produces a voltage drop under load that must be included in the measurement:
Cable voltage drop at 20A through 10m of 2.5mm² cable:
V_drop = I x R = 20A x (0.0175 x 20m / 2.5mm²) = 20A x 0.14Ω = 2.8V
A generator producing 192V at output terminals delivers 189V at inverter input.
189V > 180V acceptance threshold but below 210V stable operation minimum.
-> Accepted initially, rejected after sustained low-voltage monitoring period.
Conventional generators with brush AVRs produce voltage regulation of ±5 to ±10 percent under steady load and ±15 to ±20 percent during load transients. Inverter generators produce voltage regulation of ±1 percent and THD below 3 percent regardless of load, making them the correct specification for installations serving sensitive electronics including computers, servers, and variable speed drives.
Generator Power Quality Acceptance Checklist:
Voltage at inverter terminals under combined load -> 210V to 250V for stable operation
Voltage measurement point -> Multiplus-II AC input terminals, not generator output
Frequency under full load -> 49.5Hz to 50.5Hz for stable operation
AVR type -> electronic AVR preferred; mechanical governor not acceptable
THD at inverter terminals -> below 5% general loads, below 3% sensitive electronics
Inverter generator preferred -> Honda EU, Yamaha EF, or equivalent for sensitive loads
Cable voltage drop -> calculate and verify within acceptable limit at full load
Test procedure -> measure V and f at inverter terminals under combined load
Automatic Generator Start Configuration
The Cerbo GX’s generator start/stop function provides automatic generator control through a two-wire connection between the Cerbo GX’s dry contact relay output and the generator’s remote start input terminals. The relay closes to start the generator and opens to stop it. Five trigger conditions are available: SoC below threshold, battery voltage below threshold, AC load above threshold, time-based schedule, and manual override. Multiple conditions can be active simultaneously and the generator starts when any enabled condition is met.
The recommended configuration for the cluster system balances fuel efficiency, battery longevity, and generator service life:
Auto-Start Configuration — Cluster System:
Start conditions:
SoC trigger: start when SoC < 30% for 5 continuous minutes
Voltage trigger: start when battery voltage < 46.4V for 2 continuous minutes
Stop conditions:
SoC target: stop when SoC > 80%
Minimum run time: 30 minutes must elapse before stop condition is evaluated
Failure alarm: VRM alert if generator fails to start after 3 consecutive attempts
The 30% SoC start trigger maintains a minimum one-day autonomy reserve before the generator starts. At the cluster system’s daily discharge of 21,321Wh and usable capacity of 51,120Wh, 30% SoC corresponds to 15,336Wh of remaining capacity — sufficient to serve one full worst-case day before depletion.
The 30-minute minimum run time is the most important stop condition parameter and the one most consistently omitted. Without it, the Cerbo GX may stop the generator within 10 to 15 minutes on a system where solar is producing significant power at the time of generator start. Short-cycling accumulates start wear at many times the rate of a sustained run and consumes more fuel per kWh delivered because the generator spends a disproportionate fraction of its run time in the inefficient warm-up phase.
Auto-Start Configuration Checklist:
SoC start trigger -> 30% SoC for 5 continuous minutes
Voltage start trigger -> 46.4V for 2 continuous minutes (backup condition)
SoC stop target -> 80% SoC
Minimum run time -> 30 minutes before stop condition evaluated
Manual override -> enabled on GX Touch and VRM portal
Start failure alarm -> VRM alert after 3 failed start attempts
Run hour alert -> VRM alert at 240 cumulative hours for oil change reminder
Hour counter reset -> manual reset in Cerbo GX generator menu after each service
Hybrid Operating Modes on the Victron Platform
The Victron Multiplus-II supports four distinct operating modes that determine how it manages the relationship between the AC input source, the battery bank, and the connected AC loads.
Inverter-Only Mode
In inverter-only mode the Multiplus-II operates exclusively from the battery bank. The AC input terminal is either disconnected or disabled in VEConfigure. The solar array charges the battery through the MPPT controller and the inverter serves the loads from the battery. This is the default operating mode for a pure off-grid system with no generator connected and the normal daytime operating condition on the cluster system when the battery is above the generator start threshold.
Passthrough Mode
In passthrough mode the Multiplus-II has accepted a valid AC source at its input terminal and transferred the connected loads to the AC input source. The AC source simultaneously serves the loads and provides power to the built-in charger. The inverter stage is bypassed for the connected loads during passthrough, reducing inverter losses. The charger draws current from the AC source up to the AC input current limit. Passthrough mode is the normal generator operating condition on the cluster system.
PowerAssist Mode
PowerAssist mode activates when the connected AC load exceeds the AC input current limit. The Multiplus-II supplements the generator’s output with battery discharge current, adding inverter output to the generator’s passthrough output to serve the combined load. The generator operates at or below the AC input current limit throughout the PowerAssist event:
PowerAssist Example — Cluster System:
AC input current limit: 8A
Generator provides: 8A x 230V = 1,840W
Running load: 1,950W -> inverter adds 110W from battery continuously
Pump starting surge (with soft starter, 3x inrush):
Total during pump start: 1,950W + 4,500W = 6,450W
Generator contributes: 1,840W (at 8A limit)
Inverter adds: 6,450W - 1,840W = 4,610W from battery during surge
Battery current during surge: 4,610W / 48V = 96.0A < DCL of 148A -> PASS
Duration: 2 to 3 seconds during soft-start ramp
ESS Mode
ESS mode is the Victron Energy Storage System mode designed for grid-connected hybrid installations. It requires a grid connection and is not applicable to the off-grid cluster system. It is noted here because it appears in VEConfigure alongside the other operating modes and is sometimes incorrectly selected on off-grid installations, producing unexpected behaviour when the inverter cannot detect the grid reference it expects.
Operating Mode Selection Reference:
Inverter-only mode -> normal daytime operation, no generator or below start threshold
Passthrough mode -> generator running, loads served from generator, battery charging
PowerAssist mode -> generator at input limit, inverter supplements for load peaks
ESS mode -> grid-connected hybrid only — do NOT select on off-grid systems
Generator Run Time Optimisation
The generator run time optimisation problem balances three competing costs: the fuel cost of generator operation, the battery degradation cost of deep cycling, and the generator wear cost of frequent starts. The three configuration variables that determine run time behaviour are the SoC start trigger, the SoC stop trigger, and the minimum run time.
Run Time Optimisation Variables:
SoC start trigger effect:
Low trigger (20%) -> fewer starts, deeper discharge, higher battery degradation
High trigger (50%) -> more starts, shallower discharge, lower degradation, more fuel
Optimal (30%) -> balances fuel cost and battery degradation at cluster system DoD
SoC stop trigger effect:
Low stop (60%) -> shorter runs, less fuel per run, more frequent restarts
High stop (90%) -> longer runs, more fuel, fewer restarts, full recharge
Optimal (80%) -> leaves 20% for solar to complete charge next morning
Minimum run time: 30 minutes as established in Section 4
The break-even analysis between fuel cost and battery degradation cost confirms the 30 percent start trigger:
Fuel cost per run cycle:
Generator at 75% load: 1.2L/hr x 0.75 = 0.90L/hr
Minimum run time: 0.5hr
Fuel per cycle: 0.45L x ₦1,400/L = ₦630
Battery degradation cost per deep cycle:
Replacement cost: ₦2,800,000 / 10,000 rated cycles = ₦280 per cycle
At 55% DoD (30% trigger): ~₦420 per cycle (interpolated)
Break-even: fuel saving (₦630) > degradation cost (₦420)
Net saving: ₦210 per event in favour of 30% trigger vs 50% trigger
Run Time Optimisation Reference:
SoC start trigger -> 30% (balances fuel cost and battery degradation)
SoC stop trigger -> 80% (leaves headroom for solar completion)
Minimum run time -> 30 minutes (prevents short-cycling)
Seasonal adjustment -> raise to 35% during harmattan (reduced solar yield)
Rainy season note -> increase minimum run time to 45 minutes in extended overcast
Three-Phase Generator Integration
Three-phase generator integration on the commercial system from Post #16 adds three verification requirements beyond the single-phase framework: phase sequence verification, per-phase AC input current limit configuration, and load balance measurement.
Phase Sequence Verification
The three-unit Multiplus-II configuration in three-phase mode expects the generator to deliver phases in the sequence L1-L2-L3 in standard IEC rotation. A reversed sequence L1-L3-L2 causes all three units to reject the source simultaneously. The Multiplus-II does not produce a specific phase sequence alarm — it simply fails to accept the generator source, indistinguishable from a voltage or frequency rejection without a phase sequence measurement. Phase sequence verification requires a phase sequence indicator connected at the Multiplus-II AC input terminals before the generator is started under load. A reversed sequence is corrected by swapping two phase connections at the generator terminal block.
Per-Phase AC Input Current Limit
The AC input current limit on a three-phase system is configured independently per unit in VEConfigure. Each unit’s limit governs the current drawn from its assigned phase:
Per-phase AC input limit for commercial system (8kVA generator):
Generator safe current per phase: (8,000VA / 3 / 230V) x 0.80 = 9.27A
Phase A: load 5.7A, available for charging: 9.27 - 5.7 = 3.57A -> set to 3A
Phase B: load 9.4A, exceeds per-phase capacity -> PowerAssist from battery
Phase C: load 14.0A, exceeds per-phase capacity -> PowerAssist from battery
Conclusion: load asymmetry requires generator sized for Phase C peak, not average.
Phases B and C rely on PowerAssist during generator operation.
Load Balance Measurement
After configuring per-phase input limits and starting the generator, AC current on each phase must be measured at the Multiplus-II AC input terminals under the actual building load. A phase imbalance above 20 percent between the most and least loaded phase indicates load distribution requires adjustment.
Three-Phase Generator Integration Checklist:
Phase sequence check -> verified at inverter input terminals before first start
Phase sequence tool -> rotating disc or LED indicator, mandatory commissioning equipment
Reversed sequence fix -> swap two phase connections at generator terminal block
Per-phase input limit -> calculated and set independently per unit in VEConfigure
Load balance measurement -> AC current per phase measured under combined load, imbalance < 20%
For the three-phase inverter configuration and load distribution, refer to our engineering guide on how to size an off-grid solar system for commercial buildings.
Generator Maintenance Integration
The Cerbo GX accumulates generator run hours automatically through the start/stop relay connection, incrementing a cumulative hour counter each time the relay closes and stopping when it opens. This provides the data for maintenance scheduling without requiring the client to track hours manually, which on a system with automatic generator start produces a run hour accumulation that no client reliably tracks without a dedicated instrument.
Configuring VRM alerts at 240, 490, and 990 cumulative hours provides an automatic maintenance reminder before each service interval is exceeded, with a 10-hour buffer that accommodates the time between alert and technician visit. The maintenance log entry in the commissioning documentation after each service creates a traceable history that supports warranty claims on both the generator and the battery bank.
Generator Run Hour Alert Configuration:
240-hour alert -> oil change due within 10 hours; schedule service immediately
490-hour alert -> spark plug, air filter, oil change; schedule combined service
990-hour alert -> full scheduled service including carburetor
Hour counter reset -> manual reset in Cerbo GX generator menu after each service
Service log entry -> record hour counter, date, parts replaced, technician name
Start failure log -> Cerbo GX records failed auto-start attempts; review monthly in VRM
Conclusion
The generator that ran for 85 seconds every evening and delivered nothing was not a faulty generator. It was a correctly functioning generator connected to a system that had not been verified under the actual load conditions it would encounter in service. The voltage sag that caused the Multiplus-II to reject the source was present on the first day of operation and every day thereafter. It was detectable with a voltmeter and two minutes of measurement time at the inverter’s AC input terminals under the combined load. It was not detected because the commissioning procedure did not include that measurement.
The three parameters that determine whether generator integration works correctly are the AC input current limit, the generator start/stop configuration, and the power quality verification under load. All three must be correct simultaneously. A correctly sized generator with the wrong AC input current limit will overload on the first automatic start. A correctly configured AC input current limit with a generator that cannot maintain voltage under the combined load will produce the 85-second failure cycle from the introduction. A correctly sized and configured system with no minimum run time will short-cycle and accumulate start wear that shortens the generator’s service life regardless of how well it is maintained.
The framework in this post resolves all three failure modes. Apply Section 1 to size the generator against the combined demand. Apply Section 2 to configure the AC input current limit correctly. Apply Section 3 to verify power quality at the inverter terminals under load before commissioning sign-off. Apply Section 4 to configure auto-start with the correct SoC trigger, stop condition, and minimum run time. The generator that was delivering nothing will deliver reliably.
In the next post we examine the most common off-grid system mistakes across design, installation, and commissioning — a consolidated field guide to the errors that produce callbacks, warranty disputes, and premature component replacement.
For the inverter configuration, battery bank specifications, and commissioning framework that generator integration builds on, refer to our engineering guides on how to size and select an off-grid inverter — complete worked example, battery bank sizing, configuration, and BMS selection, system monitoring, commissioning, and fault diagnosis, and off-grid system sizing for small commercial and light industrial loads.
I am Engr. Ubokobong Ekpenyong, a solar specialist and lithium battery systems engineer with over five years of hands-on experience designing, assembling, and commissioning off-grid solar and energy storage systems. My work focuses on lithium battery pack architecture, BMS configuration, and system reliability in off-grid and high-demand environments.