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The IV curve: Voc, Isc and the knee

Solar PV physics · Intermediate · about 12 minutes · part of Solar PV Systems

A panel is not a voltage source. Open, it shows Voc and gives nothing; shorted, it gives Isc and no volts; the power is at the knee, Vmp × Imp. Sweep the load and the operating point slides along the curve.

The IV curve: Voc, Isc and the knee: the circuit on the simulator's bench

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How it works

At 0 W/m² this module's curve has Voc 0.0 V, Isc 0.00 A and its knee at 0.0 V × 0.00 A = 0 W. It is running at 0.0 V, 0.00 A: 0 W.

Open the IV / PV curve card on the panel and drag the load's resistance: the dot slides along the curve. Only one resistance lands on the knee; that is the job an MPPT controller does by itself.

Try it on the bench

  1. Select the load and raise its resistance to 100 Ω: the voltage climbs to Voc and the power falls to almost nothing.
  2. Lower it to 0.5 Ω: the current is Isc and the power is tiny again.
  3. Set the panel to 500 W/m²: Isc halves, Voc barely moves.
  4. Switch to the 400 W panel: 72 cells, twice the voltage.

Parts on the bench

In the course

Solar PV Systems, module “1 · PV basics”.

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