Off-Grid Startup Guide

D-Cube-H125 / H261 + Solis 125K — Off-Grid / Backup / Generator / Paralleling

Document version: V1.0 (2026-08-26) Scope: BESS selection / bidding / engineering implementation for EU off-grid + backup + generator scenarios Hybrid inverter: Solis S6-EH3P125K10-NV-YD-H (125 kW)

1. Three Off-Grid Application Modes

2. Backup Output — 125 kW Continuous + < 10 ms Transfer

2.1 Key Facts

• 125 kW is continuous rated, not peak
• < 10 ms transfer certified by G99 Type B test (TÜV SÜD report 704092514279-00)

2.2 125K Backup Parameters

2.3 Transfer Time Details

• Grid-tied → Off-grid (grid loss): < 10 ms (automatic)
• Off-grid → Grid-tied (grid restoration): Wait for V/f stabilization, then synchronize (tens of seconds, depending on grid stability)
• PV → Battery: Internal DC-bus transfer, no interruption
• Generator → Battery: Controlled by SOC threshold, see §7

3. Overload Curve

3.1 125K Overload Capability

3.2 Design Consequences

• ❌ Customer starts 200 kW motor load, single 125K will trip (even at 10 s)
• ❌ Alarm Over-Load or OV-ILLC (user manual p.78, 81)
• ✅ 125K starting 200 kW locked-rotor load → must split into 2 units

3.3 Design Margin for Key Application Scenarios

4. Black-Start Matrix

4.1 Comparison of 5 Startup Sources

5. Off-Grid Operating Rules

5.1 User Manual

1. PV power flow priority: load > battery (no export to grid)
2. PV > load + battery charging capacity: inverter hard-clips PV (MPPT direct limiting, not frequency-shift derating like AC-coupled inverters)
3. AC-coupled PV inverter (if any): must be configured to "AC coupling" mode + frequency trip value > Solis over-frequency threshold

5.2 Over-Frequency Threshold Settings (50 Hz Grid)

• Why it matters: If the AC-coupled PV inverter is not set to "AC coupling" mode during off-grid operation, it may form a frequency loop with Solis, causing mutual inverter interference and system collapse.

5.3 Off-Grid Power Balance Example

Assumed H125 cabinet configuration:

• Battery capacity: 261 kWh × 95 % DoD = 247.95 kWh usable
• Average load: 50 kW (daytime) / 80 kW (night)
• PV peak: 100 kWp (single inverter supports up to 250 kWp)
• Battery max discharge: 125 kW (single unit)

Typical daily energy flow:

6. Black-Start Hard Limits

6.1 Four Off-Grid Startup Designs That Must Be Avoided

❌ 1. No 100 % Motor-Load Black-Start

• The backup mode overload curve also applies
• 125K cannot start 200 kW locked-rotor load from zero
• Design: Use VFD for large motors, or split into 2 × 125K

❌ 2. No Built-in Transformer

• Solis 125K generates 230/400 V directly from the battery DC bus via IGBT
• Not an isolation transformer output
• Sensitive loads (large UPS / medical equipment) require an external isolation transformer

❌ 3. No PV-Only Battery-less Black-Start

• Not supported (see §4)
• Must configure battery or backup generator

❌ 4. No Single-Unit Black-Start (parallel scenario)

• For multi-unit paralleling, power up Master first, then power up Slaves sequentially (user manual §3.9.7 p.35)
• Slave will not automatically re-elect a Master

7. Diesel Generator Integration

7.1 Hardware Interface

The inverter exposes 2 DO + 2 DI dry contacts:

7.2 Auto-Start Control Logic

Key fact: The generator is controlled by battery SOC threshold, not grid status.

⚠️ ON SOC must be < OFF SOC (hysteresis); otherwise the generator will cycle on/off. Solis minimum hysteresis = 15 percentage points (e.g., ON = 20 %, OFF = 35 %).

UK winter low-temperature + high-load scenario: recommended hysteresis ≥ 20 % — otherwise the generator may start/stop 5–10 times per day, accelerating wear.

7.3 Three Smart Port Modes

Typical multi-mode combination: Generator input + smart load output enabled simultaneously (supported by Solis); generate power in off-grid mode, smart load balances PV surplus in grid-tied mode.

7.4 Generator Sizing Rules

7.5 Sizing Decision Matrix

8. Generator Start/Stop Signal Chain

8.1 Detailed Step-by-Step Process

8.2 Time Budget (Typical)

Total: SOC drops to ON → generator takes over load: ~40 seconds Total: SOC charges to OFF → generator stops: ~5–30 minutes

9. Multi-Unit Paralleling

9.1 Parallel Limit

✅ Up to 10 units Solis 125K can be paralleled with each other (grid-tied + off-grid) 10 × D-Cube-H125/H261 = 1.25 MW AC / 2.61 MWh per site For more than 6 units, Solis STS cabinet is recommended

9.2 Master-Slave Configuration

9.3 Parallel Communication

• Physical link: P-A, P-B shielded CAT5 cable
• Shared information:
• PV voltage / current per MPPT
• Battery SOC / power
• Load demand
• Grid-tied / power intake
• Dispatch commands (from Master)
• Topology: Entire cluster = single BESS to upper-layer EMS

9.4 Special Considerations for Off-Grid Paralleling

11. Common Design Mistakes

• ❌ Mistake 1: Using the brochure's "2× / 200 ms" directly for motor load design

  Correct: 125K does not support 2×; the upper limit is 1.6× / 200 ms. Designing 200 kW motor load requires splitting into 2 × 125K or using VFD.

• ❌ Mistake 2: "PV-only, no battery" configuration as a black-start solution

  Correct: The Solis 125K control board is powered from the battery DC bus; PV-only with no battery cannot start. Must configure battery or backup generator.

• ❌ Mistake 3: SOC threshold ON > OFF or hysteresis < 15 %

  Correct: ON must be < OFF, hysteresis ≥ 15 % (Solis minimum). UK winter recommendation ≥ 20 %. Otherwise the generator cycles on/off, 5–10 times per day, scrap in half a year.

• ❌ Mistake 4: Generator grounding connected to inverter enclosure

  Correct: User manual p.37 explicit — generator chassis independently grounded + PE bar (not inverter enclosure). Wrong connection causes circulating current, triggers RCD trip.

• ❌ Mistake 5: Grid breaker not opened when generator is running

  Correct: User manual p.37 explicit — must open the grid-side breaker (or RCD) when the generator is running. Otherwise backfeed to the grid, illegal + safety incident.

• ❌ Mistake 6: AC-coupled PV inverter not configured to "AC coupling" mode

  Correct: If not configured to AC coupling mode during off-grid operation, may form a frequency loop with Solis, causing system collapse. Must configure AC coupling + frequency trip > Solis over-frequency threshold (50.1–54 Hz).

• ❌ Mistake 7: Designing PV > 250 kWp (single 125K)

  Correct: DC/AC > 2.0× is not supported, only peak clipping. 250 kWp is the design limit. Customer wants 260 kWp must use 2 × 125K.

• ❌ Mistake 8: Assuming Slave can automatically take over Master failure

  Correct: When Master is offline, Slave will not automatically take over, system shuts down. Requires on-site manual Master restart, then sequential Slave power-up.

• ❌ Mistake 9: Using 80K / 100K overload curve for 125K

  Correct: 125K has much weaker overload capability than 80K / 100K. Bidding technical specifications must distinguish. Brochure marketing "2×" applies only to 75–100K.

• ❌ Mistake 10: In off-grid mode, trying to export all PV output to the grid

  Correct: In off-grid mode, PV priority is load > battery, no export to grid (grid is disconnected, export is impossible). If AC-coupled PV is configured, frequency-shift coordination must be used.

Appendix A: Key Terminology