From wikipedia/Maximum_power_point_tracking:

When the batteries in an off-grid system are fully charged and PV production exceeds local loads, an MPPT can no longer operate the panel at its maximum power point as the excess power has no load to absorb it. The MPPT must then shift the PV panel operating point away from the peak power point until production exactly matches demand. (An alternative approach commonly used in spacecraft is to divert surplus PV power into a resistive load, allowing the panel to operate continuously at its peak power point.)

I can't imagine why is it desired to divert the surplus power into a resistive load instead of simply ignoring it. What is the advantage of keeping PV module operate at its maximum power point at all times?

  • \$\begingroup\$ If you just "ignore" the surplus power being generated, you will probably either increase the output voltage (so that all the power is consumed) or you will waste that surplus energy in the voltage control part of the inverter \$\endgroup\$
    – jDAQ
    Jan 7, 2020 at 19:56
  • \$\begingroup\$ @jDAQ Renewable energy design principles are...so strange when you don't have fuel consumption to worry about. \$\endgroup\$
    – DKNguyen
    Jan 7, 2020 at 20:02
  • \$\begingroup\$ No, the purpose of existence of MPPT is regulating the output voltage in the first place in order to regulate the input voltage (which makes the PV deliver the maximum power). So, an MPPT is capable of delivering no power at all. \$\endgroup\$
    – ceremcem
    Jan 7, 2020 at 20:03
  • \$\begingroup\$ @DKNguyen The interesting part is that the resistive load diversion requires an extra investment for the resistive load, additional switches for the diversion and implementing the logic. So this process is not free of charge. \$\endgroup\$
    – ceremcem
    Jan 7, 2020 at 20:13
  • \$\begingroup\$ It could be as an extra layer of safety. If the MPPT also considers that its load are batteries and needs to implement CC/CV charging, then it becomes more complex. If something goes wrong, it could cook the batteries - on a spacecraft you definitely don't want this to happen. So an extra system that monitors and enforces CC/CV charging, with a resistive grid to dump extra energy can be conceptually simpler (modularized approach with simple modules). But I don't know the true answer, I'm now curious as well. \$\endgroup\$
    – anrieff
    Jan 7, 2020 at 20:17

2 Answers 2


Spacecraft aren't air-cooled. Simple as that.

Solar cells operate in full sunlight, and get hot. Dissipating 30% of the incident power somewhere else helps keep the temperature manageable.

The unlit side tends to get very cold, for much the same reason - so there may be benefit to using the heat in another part of the satellite.

  • 1
    \$\begingroup\$ I think elaboration is required since this answer implies that the resistors wouldn't produce heat dissipating the power. \$\endgroup\$
    – DKNguyen
    Jan 7, 2020 at 20:18
  • \$\begingroup\$ So the Wikipedia claim is wrong, it's not because this is "allowing the panel to operate continuously at its peak power point", but it's "allowing the panel cool down by dissipating the PV cells' energy by the resistors". \$\endgroup\$
    – ceremcem
    Jan 7, 2020 at 20:22
  • 3
    \$\begingroup\$ Not exactly wrong ... extracting max power = dissipating min power in the panel itself. \$\endgroup\$
    – user16324
    Jan 7, 2020 at 20:24
  • 2
    \$\begingroup\$ Even in a situation where the apparatus has more heat/power than is ideal, dissipating the extra power in a resistance separate from the panel allows the power to be dissipated at a higher temperature, which will aid in radiant heat transfer away from the apparatus. \$\endgroup\$
    – K H
    Jan 7, 2020 at 23:58
  • \$\begingroup\$ @BrianDrummond The Wikipedia claim is wrong in terms of reasoning: Replace the "Surplus engery = being late", "diverting energy to the resistors = put the pedal to the metal", "operating at its maximum power = running the engine at its peak power": "When we are late to the home, we must put the pedal to the metal in order to run the engine at its peak power". When I ask "why do we want to run the engine at max power when we are late?", you answer "running engine at max power = going fast". So, it must be: "When we are late to the home, we must put the pedal to the metal in order to go faster". \$\endgroup\$
    – ceremcem
    Jan 8, 2020 at 12:20

Solar cells in open circuit while in sunlight as done in a typical MPPT charge controller will heat up faster, and their efficiency will drop compared to a typical shunt charge controller setup. Shunting the excess energy at the ideal power point will maximize the power usage while minimizing the heat they need to manage. As cooling is more difficult up there, this is the preferred setup. Their (ISS) on/off cycles and load needs are also different than typical earth systems so every bit helps.

  • \$\begingroup\$ How much heat does commonly used solar panel (in residential PV systems) produce when in open circuit compared to optimum MPPT curve? Negative electricity prices are becoming more common now, so it's interesting to know what effect it would had to stop producing electricity when required. \$\endgroup\$
    – Marki555
    Apr 25, 2023 at 8:04

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