Solid-State Transformer Math Explained: Why Enphase Stacks 44 GaN Modules per Phase for 13.8 kV to 800 VDC

132 modules, 44 per phase, a few hundred volts each. That is how Enphase turned 13.8 kV of medium-voltage AC into a steady 800 VDC in its Austin lab on October 27, 2026 (Enphase press release). No single switch ever sees 13.8 kV. This post works through the solid-state transformer (SST) math behind that number: voltage per module, current per module, what a failed module costs, and why the efficiency figure is the one data center buyers actually care about.

What happened in the last two weeks

• Oct 21: Vantage secured over 1 GW of manufacturing capacity for Heron Power's Heron Link SST, which targets more than 98.5% efficiency from medium voltage to 800 VDC (Vantage).
• Oct 26: Enphase and Infineon announced 900 V GaN bidirectional switches for SST power modules (Enphase/Infineon).
• Oct 27: Enphase demonstrated 13.8 kV AC to 800 VDC with 132 GaN modules rated 5.5 kW each, and says 34.5 kV is next, with megawatt-scale racks before the end of 2026 and customer pilots in 2027.

1. Solid-state transformer voltage sharing: why 44 modules per phase

A cascaded SST stacks low-voltage modules in series on the AC side, so each phase leg splits the phase voltage. For a 13.8 kV line-to-line grid:

V_phase,rms = 13,800 / √3 ≈ 7,967 V
V_phase,peak = 7,967 × √2 ≈ 11,267 V
V_module,peak = 11,267 / 44 ≈ 256 V  (≈ 181 V rms)

That lines up with Enphase's own wording that each module "handled a few hundred volts." It also explains why the company can build these with microinverter-class GaN parts rated below 1 kV: the module-level voltage is in the same range as a solar inverter, not a utility substation.

2. Current per module: the series string sets the amps

Every module in a phase string carries the same line current. Taking the nameplate figures from the release:

P_nameplate = 132 × 5.5 kW = 726 kW
I_line = P / (√3 × V_LL) = 726,000 / (1.732 × 13,800) ≈ 30.4 A
Check per module: 5,500 W / 181 V ≈ 30.4 A
DC side: 726 kW / 800 V ≈ 908 A

So the medium-voltage side is a modest 30 A string, while the 800 VDC side has to carry roughly 900 A. That asymmetry is the whole point of the topology: high voltage stays where currents are small, and the high-current bus stays short and close to the racks. Enphase did not say what power level the demo actually ran at, so the 726 kW figure is the nameplate sum, not a measured result.

3. Module failure and redundancy: what one bypassed cell costs

Enphase says IQ SST is designed to keep running if a module fails. In a cascaded string, a failed cell is typically bypassed and the survivors absorb its share:

1 module out: 11,267 / 43 ≈ 262 V peak per module (+2.3%)
2 modules out: 11,267 / 42 ≈ 268 V peak per module (+4.8%)
Phase capacity with 1 out: 43 × 5.5 kW = 236.5 kW (vs 242 kW)

The voltage bump is small, but it eats into the same headroom you need for grid overvoltage. That is why the redundancy count and the switch voltage rating are one design decision, not two. The real bypass method and spare count per phase are not public.

Key point: In a cascaded SST, module count per phase is set by peak phase voltage divided by the voltage each module can safely hold, plus spares. Raise the grid voltage and you add modules; raise the switch rating and you remove them.

4. Scaling to 34.5 kV and the 900 V GaN shortcut

At the same ~256 V peak per module, a 34.5 kV feed needs:

V_phase,peak = 34,500 / √3 × √2 ≈ 28,170 V
Modules per phase ≈ 28,170 / 256 ≈ 110  (≈ 330 total)

This is our own extrapolation, not an Enphase figure. The 900 V GaN bidirectional switch is the lever that shrinks it: Enphase and Infineon say higher-voltage switches let a medium-voltage input be handled "by fewer inverters connected in series." If per-module voltage scaled with the 650 V to 900 V jump (a ratio of about 1.38), the 44-module string would drop to roughly 32 modules. Treat that as an illustration only.

Efficiency is where buyers look next. Heron's target of more than 98.5% from MV to 800 VDC means about 15 kW of loss per MW delivered. Against an illustrative legacy chain at 95% (MV transformer, UPS and rectifier stages; your site will differ), that is roughly 35 kW saved per MW, or about 300 MWh per year per MW of continuous load. If you want the power-factor side of this argument, we covered it in Efficiency vs. Power Factor: The Real Reason AI Data Centers Need SSTs on YouTube, and the rack-side conversion in 800V to 12V GaN PDB explained.

To verify: the actual power level of Enphase's 13.8 kV test; the module count and switch derating Enphase will use at 34.5 kV; how many spare modules per phase IQ SST carries; and whether Heron's 98.5% target is measured at full or partial load.

What to take away

• A cascaded solid-state transformer is a voltage-division problem: peak phase volts divided by safe volts per module, plus spares.
• The medium-voltage side runs tens of amps; the 800 VDC side runs hundreds. Busbar and protection design lives on the DC side.
• Higher-voltage GaN means fewer modules, fewer gate drivers and fewer failure points, which is why the 900 V switch news matters as much as the demo.
• Compare SST vendors on measured efficiency at partial load and on redundancy per phase, not just on headline kV.