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TS4 Compatibility with Bifacial PV Modules

  • Updated

Applies to: All Tigo TS4 units
Relevant Standards: NEC 690.8, IEC 61215, UL 1741, UL 3703

Overview

Bifacial PV modules can generate current from both the front and rear sides. This additional rear-side contribution increases the total current passing through Tigo’s TS4 optimizers and rapid shutdown units. 

If not properly accounted for, bifacial gain can exceed the TS4’s input current rating, leading to overheating, shutdowns, or damage.

This article explains how to calculate effective current and determine TS4 compatibility with bifacial modules.

For ease of use, a calculator is available that can help determine combability between PV module and TS4 type, including bifacial gain considerations. Click here to check it out. 

 

Step 1: Know the TS4 Maximum Input Current

TS4 Model Max input power Maximum Isc Maximum Imp
TS4-A-O 725 725W 22A 16A
TS4-A-S 725 725W 22A 16A
TS4-A-F 725 725W 22A 16A
TS4-A-2F 20A 500W per input (1000W total)  20A 15A
TS4-A-2F 25A 700W per input (1400W total) 25A 20A
TS4-X-O 800W 25A 20A
TS4-X-S 800W 25A 20A
TS4-X-F 800W 25A 20A

The current rating is the maximum current the TS4 can safely handle, and it must not be exceeded under any expected irradiance conditions.

 

Step 2: Understand the Bifacial Module Datasheet

Bifacial modules often list:

  • Front-side Isc (short circuit current under STC)
  • Rear-side Isc or bifaciality factor (e.g., “bifaciality: 80 ±10%”)

If rear-side Isc is not listed, estimate it using the bifaciality percentage and expected irradiance.

 

Step 3: Estimate the Effective Power, Isc and Imp

Use this formula:

Effective Pmax = Front Pmax × (1 + Estimated Rear Gain %)
Effective Isc = Front Isc × (1 + Estimated Rear Gain %)
Effective Imp = Front Imp × (1 + Estimated Rear Gain %)

Example:

From a SEG-640W bifacial module:

  • Pmax: 640W
  • Front Isc: 13.98A
  • Front Imp: 13.33A
  • Estimated Rear Gain: 25% (typical for high-albedo surfaces)
Effective Pmax = 640W x 1.25 = 800W
Effective Isc = 13.98A × 1.25 = 17.5A
Effective Imp = 13.33A x 1.25 = 16.7A

 

Step 4: Compare to TS4 Ratings

Using the example above:

  • TS4-A-O: 
    • Pmax 725W rating vs 800W expected (not compatible) 
    • Isc 22A rating vs 17.5A (compatible)
    • Imp 16A rating vs 16.7A (not compatible)
    • Conclusion: Not compatible
  • TS4-X-O:
    • Pmax 800W rating vs 800W expected (compatible) 
    • Isc 25A rating vs 17.5A (compatible)
    • Imp 20A rating vs 16.7A (compatible)
    • Conclusion: Compatible

Always choose a TS4 with a higher current rating than your worst-case expected paramters, including rear-side contribution.

 

Best Practices

  • Reference actual irradiance modeling when available (e.g., PVsyst rear gain simulations).
  • Never assume rear current is "free" — it counts toward the TS4 current limit.

 

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