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.
Step 1: Know the TS4 Maximum Input Current
| TS4 Model | Max Input Current |
|---|---|
| TS4-A-O | 15A, or 22A (725W model) |
| TS4-A-S | 15A, or 22A (725W model) |
| TS4-A-F/2F | 20A or 25A versions |
| TS4-X-O | 25A |
| TS4-X-S | 25A |
| TS4-X-F | 25A |
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 Isc
Use this formula:
Effective Isc = Front Isc × (1 + Estimated Rear Gain %)
Example:
From a SEG-640W bifacial module:
- Front Isc: 13.98A
- Estimated Rear Gain: 25% (typical for high-albedo surfaces)
Effective Isc = 13.98A × 1.25 = 17.475A
Step 4: Compare to TS4 Ratings
Using the example above:
- TS4-A-O (15A max): Not compatible
- TS4-X-O (25A max): Compatible
Always choose a TS4 with a higher current rating than your worst-case expected Isc, including rear-side contribution.
Best Practices
- Do not pair bifacial modules with TS4-A-O unless rear gain is negligible.
- Use TS4-X-O for high-current bifacial modules or high-albedo install environments.
- 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.