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MPPT Fundamentals

Maximum Power Point Tracking (MPPT) is the core function of any solar inverter. Understanding it is fundamental to understanding why string systems underperform in shade and how to diagnose production issues.

The I-V Curve

A solar panel produces a range of voltage and current combinations. At any moment, there is one specific voltage where power output (P = V × I) is maximized. This is the Maximum Power Point (MPP).

         Current (I)
            │
     Isc ──┤ (short circuit current)
            │╲
            │  ╲
            │    ╲
            │      ╲
            │        ╲──── MPP (max power point)
            │               ╲
            │                 ╲
            │                   ╲
     0 ─────┼────────────────────┤── Voltage (V)
                                Voc (open circuit voltage)

The MPPT algorithm continuously adjusts the operating voltage to stay at the peak of this curve as conditions change (irradiance, temperature, partial shading).

How MPPT Works in Practice

  1. Inverter measures current output at its operating voltage
  2. Slightly adjusts voltage (higher or lower)
  3. Measures whether power increased or decreased
  4. Moves toward the higher power point
  5. Repeats continuously (typically every few seconds)

This perturb-and-observe method is simple and effective under stable conditions. Under rapidly changing conditions (fast-moving clouds, shifting shade), the algorithm may briefly track sub-optimally before recovering.

Why Shade Breaks String MPPT

When one panel in a series string is shaded:

  • That panel's current drops dramatically
  • Current in a series circuit is equal throughout the string
  • The entire string's current is dragged down to the shaded panel's level
  • MPPT can only find the maximum power point for the whole string — it cannot compensate for individual panel differences

Result: One shaded panel can reduce an entire string's output by 50% or more.

The bypass diode solution: Panels have bypass diodes across each cell group (typically 3 per panel). When shade activates a bypass diode, that cell group is bypassed — the string continues at reduced current rather than collapsing entirely. This creates a second (lower) power point on the I-V curve, which the MPPT must find.

Temperature Effects on MPPT

Parameter Effect of Temperature Increase
Voc (open circuit voltage) Decreases (~0.35%/°C for silicon)
Isc (short circuit current) Slightly increases
MPP Voltage Decreases
MPP Power Decreases (~0.5%/°C for silicon)

Field implication: String voltage is highest on cold, clear days. Verify that winter Voc does not exceed the inverter's maximum DC input voltage. Overvoltage can damage the inverter.

SMA MPPT Range

SMA inverters specify: - Vmpp min/max — the operating MPPT range - Voc max — absolute maximum input voltage (safety limit) - Isc max — maximum input current per string

The string must be designed to stay within these limits across the expected temperature range. In the field: if Voc is close to the maximum on a cold morning, the inverter may shut down until voltage drops. This is a design issue, not a fault.