SolaX Phase Envelope Protection

Author: fxstein
Date: January 9, 2026
GitHub: https://github.com/fxstein/homeassistant-solax-modbus

Applies To: X3 (Three-phase) inverters with remote control

Overview

Phase envelope protection prevents individual phases from exceeding the fuse limit (e.g., 63A) when phase imbalance exists due to single-phase loads like EV chargers. This feature automatically limits import power to protect against fuse overload while maximizing system capacity.

The Problem

Phase Imbalance from Single-Phase Loads

When you have single-phase loads (e.g., a 16A EV charger on L1), the phases become imbalanced:

Phase Current Visualization (0A ─────────────────────>| 63A)
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
                                           59.85A |   | 63A
                                       (95% Safe) |   | (Fuse)
                                                  ↓   ↓

SCENARIO 1: Without Imports (Battery Hold) - House Load Only
─────────────────────────────────────────────────────────────────
L1: [====4A====][======Rivian 16A======]··········|   20A (40A to limit)
L2: [====4A====]··································|   4A  (56A to limit)
L3: [====4A====]··································|   4A  (56A to limit)
    House load: 4A base per phase + Rivian 16A on L1 = 6.4kW total
    └─ Imbalance: 16A (entirely from single-phase EV charger)

SCENARIO 2: Requesting 30kW Import (43.9A per phase) - UNSAFE!
─────────────────────────────────────────────────────────────────
Target: 30kW ap_target + 6.4kW house = 36.4kW total import
Import per phase: 30,000W / (3 × 228V) = 43.9A per phase

L1: [====4A====][======16A======][========43.9A====XXXXX] 64A ✗ EXCEEDS!
L2: [====4A====][==========43.9A==========]········|   48A ✓
L3: [====4A====][==========43.9A==========]········|   48A ✓

    Limit Check 1 - Import Limit:
    • Total import: 36.4kW
    • Import limit: 35kW
    • Safe ap_target: 35kW - 6.4kW = 28.6kW ✓ (30kW request reduced by import limit)

    Limit Check 2 - Phase Limit (MORE RESTRICTIVE):
    • L1: 20A house + 43.9A import = 63.9A (EXCEEDS 63A fuse, 59.85A safe limit)
    • Safe ap_target: (59.85A - 20A) × 3 × 228V = 27.2kW ✗ (MORE RESTRICTIVE)

    └─ REDUCED: Phase limit (27.2kW) would be more restrictive than import limit (28.6kW)

SCENARIO 3: Requesting 30kW with Phase-Protected 27.2kW Import (39.8A per phase) - SAFE ✓
─────────────────────────────────────────────────────────────────
Same as SCENARIO 2 but now with phase protection active:

Limit Check 1 - Import Limit:
  Safe ap_target: 35kW - 6.4kW = 28.6kW ✓ (RESTRICTIVE)

Limit Check 2 - Phase Limit (CONSTRAINING):
  L1 can accept: (59.85A - 20A) = 39.85A more
  Safe ap_target: 39.85A × 3 × 228V = 27.2kW ✗ (MORE RESTRICTIVE)

Result: ap_target = min(30kW, 28.6kW, 27.2kW) = 27.2kW

L1: [====4A====][======16A======][========39.8A=======]|  59.8A ✓ (0.05A to limit!)
L2: [====4A====][========39.8A========]················|  43.8A ✓ (16A to limit)
L3: [====4A====][========39.8A========]················|  43.8A ✓ (16A to limit)

    Analysis:
    • L1: 20A house + 39.8A import = 59.8A (just below 59.85A safe limit) ✓
    • L2/L3: 4A house + 39.8A import = 43.8A (16A below limit) ✓
    • Total: 27.2kW + 6.4kW = 33.6kW (within 35kW import limit) ✓
    • Constrained by: PHASE LIMIT (27.2kW), not import limit (28.6kW)
    └─ SUCCESS: Phase protection prevents L1 from exceeding fuse limit

Key Points:
• House load imbalance (16A) is FIXED regardless of import level
• Inverters balance imports: each phase gets exactly ap_target / 3
• Must respect BOTH constraints: 95% fuse limit (59.85A) AND import limit (35kW)
• System uses the MORE RESTRICTIVE of the two limits
• In this scenario: PHASE LIMIT (27.2kW) is more restrictive than import limit (28.6kW)

The Challenge

Without phase protection: - Requesting 30kW import with 6kW house load (16A on L1) - Inverters distribute 30kW evenly: 43.9A per phase - L1 total: 16A + 43.9A = 59.9A (exceeds 59.85A safe limit) - Risk of fuse blow on L1

The Solution

How It Works

Measure Phase Imbalance
Uses measured_power_l1/l2/l3 to detect imbalance
Imbalance in measured power = imbalance in house load (inverters balance imports)
Calculate House Load Per Phase house_load_l1 = (house_load / 3) + (avg_measured_power - measured_power_l1)
Extracts actual house load distribution from measurements
Works regardless of current import level
Calculate Safe Import safe_ap_target = (59.85A - worst_phase_house_current) × 3 × avg_voltage
Ensures worst phase stays below 59.85A (95% of fuse limit)
Allows maximum import while protecting phases
Apply Both Limits ap_target = min(desired, import_limit - house_load, safe_ap_target)
Respects both total import limit AND phase limit
Uses the more restrictive of the two

Key Insight

All imbalance comes from house load - inverters distribute imports perfectly: - Each phase gets exactly ap_target / 3 of the import current - The imbalance (difference between phases) is FIXED by house load - This allows accurate calculation without feedback loops

Configuration

Required Sensors

Phase protection requires these sensors (available on X3 inverters): - measured_power_l1/l2/l3 - Phase-specific grid power (W) - grid_voltage_l1/l2/l3 - Phase-specific voltages (V) - main_breaker_current_limit - Fuse size setting (A, e.g., 63A)

Automatic Activation

Phase protection activates automatically when: - All required sensors are available - Main breaker current limit is configured in inverter - Remote control is active

No additional configuration needed!

Behavior

When Phase-Limited (Phase Constraint is More Restrictive)

Example: Rivian 16A on L1, 6.4kW house load (4A base + 16A Rivian on L1)

House: L1=20A, L2=4A, L3=4A (high imbalance from single-phase EV)

Import limit check:
  Safe ap_target: 35kW - 6.4kW = 28.6kW ✓

Phase limit check (MORE RESTRICTIVE):
  L1 remaining: 59.85A - 20A = 39.85A
  Safe ap_target: 39.85A × 3 × 228V = 27.2kW ✗

Result: ap_target = min(30kW, 28.6kW, 27.2kW) = 27.2kW
  Total import: 27.2kW + 6.4kW = 33.6kW ✓
  L1 actual: 20A + 39.8A = 59.8A ✓ (0.05A below safe limit)
  Constraint: PHASE LIMIT (27.2kW) - protecting L1 from fuse blow

When Import-Limited (Import Constraint is More Restrictive)

Example: No EV charging, 4kW balanced house load

House: L1=6A, L2=6A, L3=6A (no imbalance)

Import limit check (MORE RESTRICTIVE):
  Safe ap_target: 35kW - 4kW = 31kW ✗

Phase limit check:
  L1 remaining: 59.85A - 6A = 53.85A
  Safe ap_target: 53.85A × 3 × 228V = 36.8kW ✓

Result: ap_target = min(30kW, 31kW, 36.8kW) = 30kW
  Total import: 30kW + 4kW = 34kW ✓
  L1 actual: 6A + 43.9A = 49.9A ✓ (10A below safe limit)
  Constraint: Desired target (30kW) achieved, neither limit constraining

Logging

Debug Logging

Enable debug logging to see phase protection calculations:

logger:
  default: warn
  logs:
    custom_components.solax_modbus.plugin_solax: debug

Log Output

[REMOTE_CONTROL] Phase currents - Measured: L1=54.47A L2=49.50A L3=49.14A | 
                                   House: L1=9.50A L2=5.32A L3=4.39A | worst=L1
[REMOTE_CONTROL] Phase protection: L1 house=9.50A limit=59.85A remaining=50.35A 
                                   safe_ap_target=33908.5W
[REMOTE_CONTROL] Import bounds: ap_target=29999.0W import_bound=30693.0W 
                                import_limit=35000.0W house_load=4307.0W 
                                total_import=34306.0W

Benefits

Automatic Protection: No manual intervention needed
Maximizes Capacity: Allows maximum import while staying safe
Prevents Fuse Blow: Keeps worst phase below 95% of fuse limit
Stable Operation: No oscillation or feedback loops
Works with EV Charging: Handles single-phase and three-phase EV chargers
Respects Both Limits: Honors both import limit and phase limits

Technical Details

The Math

Given: - Total house_load (W) - measured_power_l1/l2/l3 (includes house + imports) - Fuse limit (e.g., 63A) - Import limit (e.g., 32kW)

Calculate:

# Extract house load per phase using imbalance
avg_measured = (measured_l1 + measured_l2 + measured_l3) / 3
house_l1 = (house_load / 3) + (avg_measured - measured_l1)

# Convert to current
house_current_l1 = house_l1 / voltage_l1

# Calculate safe ap_target
remaining_A = (fuse_limit × 0.95) - house_current_l1
safe_ap_target = remaining_A × 3 × avg_voltage

# Apply both limits
ap_target = min(desired, import_limit - house_load, safe_ap_target)

Why It Works

The imbalance in measured_power equals the imbalance in house load because inverters balance imports:

measured_l1 - measured_l2 = (house_l1 - import_l1) - (house_l2 - import_l2)
                          = house_l1 - house_l2  (imports cancel)

This allows extracting house load distribution without feedback loops.

Limitations

X3 inverters only: Requires three-phase sensors
Requires fuse setting: Must configure main_breaker_current_limit in inverter
5% margin: Targets 95% of fuse limit (brief excursions above acceptable)
Balanced import assumption: Assumes inverters distribute imports evenly (verified accurate)

Testing

Tested and verified with: - ✅ Single-phase EV charger (Rivian 16A) - ✅ Dual EV chargers (Rivian 16A + Tesla 16A 3-phase) - ✅ Parallel mode (3 inverters) - ✅ Various house loads (4kW - 18kW) - ✅ Import limits (32kW - 35kW) - ✅ Phase limits (55A - 63A)

Results: Stable operation, no oscillation, correct limiting behavior.