Imp (Maximum Power Current)

Key Takeaways

• Imp = current at maximum power point, at STC.
• Typically 0.92 × Isc.
• Used to verify inverter per-MPPT current limit.
• Bifacial Imp is 5–15% higher than front-only — account for it.
• Stays roughly constant with temperature; small +0.04 to +0.06 %/°C increase.

Why Imp Matters in String Design

Imp is the number a designer checks last, right before locking the stringing plan — not because it’s an afterthought, but because it only becomes a constraint once you know how many strings land on each MPPT input. Every string in a parallel group adds its Imp to the total current the inverter’s MPPT channel has to swallow. Go over the rated limit and the inverter either clips output or trips a protection fault; stay well under it and you’ve left MPPT capacity — and yield — on the table. That’s why Imp, alongside Vmp, sits at the center of the string-sizing checks run during detailed engineering, well after the module’s STC rating has already fixed the number on the datasheet.

Worked Example

Take a module rated 13.0 A Isc at STC. Using the typical 0.92 ratio, Imp ≈ 0.92 × 13.0 A ≈ 12.0 A. If the inverter’s per-MPPT current limit is 25 A and the design calls for two strings in parallel on that MPPT, combined current is 2 × 12.0 A = 24.0 A — inside the limit, with roughly 1 A of headroom. Switch to a bifacial module where rear-side gain pushes real-world Imp 10% higher, and that same pair of strings would draw closer to 26.4 A, tripping the 25 A ceiling. This is exactly the kind of check that string-sizing calculations catch before equipment is ordered rather than after it’s installed.

Related Reading

Imp verification is one piece of the inverter and stringing checks covered in Heaven Designs’ PV yield simulation software comparisons and in the PVsyst tracker yield study methodology, where per-MPPT current limits get modeled alongside shading and tracking losses. Module current values like Imp and Isc also show up directly on equipment schedules in solar SLD software, since single line diagrams typically call out both figures per string. For a deeper look at the control loop that actually uses Imp and Vmp in real time, Qbits Energy’s writeup on the MPPT algorithm is a useful companion piece.