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PWM against MPPT: the harvest, as a number

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

A PWM controller is a switch: it connects the panel straight to the battery, so the panel runs at battery voltage (about 13 V) instead of its 18 V knee, and the difference is thrown away. An MPPT runs the panel at the knee and converts down. Two identical panels, two identical batteries, two controllers: the Wh counters tell the story, and it is biggest on cold mornings and with high-voltage panels.

PWM against MPPT: the harvest, as a number: the circuit on the simulator's bench

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How it works

PWM: the panel sits at 0.0 V giving 0 W. MPPT: the panel sits at 0.0 V giving 0 W. Harvested so far: 0.0 Wh against 0.0 Wh.

The gap is (Vmp − Vbatt) × I: it grows when the battery is low (lower voltage), when the panel is cold (higher Vmp), and with a 72-cell panel on a 12 V battery, where PWM throws away more than half.

Try it on the bench

  1. Let it run a few minutes at this speed and compare the two Wh counters.
  2. Swap both panels for the 400 W 72-cell: PWM now holds a 41 V knee at 13 V and loses most of it.
  3. Turn the weather on at 10 °C: Vmp rises and the MPPT’s lead grows.

Parts on the bench

In the course

Solar PV Systems, module “6 · Charge controllers: PWM and MPPT”.

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