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101 renewable - maximum power point tracking

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Last Updated
7th of January, 2020

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Maximum power point tracking (MPPT) is a technique used by grid-tie inverters, solar battery chargers, and similar devices in order to get the maximum possible output power from the PV array. Solar cells have a complex relationship between solar irradiation, temperature and total resistance that produces. This relation produces a non-linear output efficiency curve known as the I-V curve. It is the purpose of the MPPT system incorporated into inverters and charge controllers to sample the output of the cells and apply a resistance (load) to obtain maximum power for any given environmental conditions. Essentially this defines the current that the inverter should draw from the PV in order to get the maximum possible power (since power equals voltage times current). And based on this indication from the MPPT system the inverters or charge controllers are actioning on the PV array output. Below are some of the MPPT conditional situations to explain how this principle will apply.

At night, an off-grid PV power system uses batteries to supply its loads. The battery bank voltage when fully charged may be close to the point where the PV array's peak power is the maxim. This is unlikely to be true at sunrise when the battery is partially discharged. Charging may begin at a voltage considerably below the array peak power-point, and an MPPT system can resolve this mismatch indicating the charger to start charging from the next closest available value on the MPPT hysteresis.

When the batteries in an off-grid system are full and PV production exceeds local loads, an MPPT can no longer operate the array at its peak power point as the excess power has nowhere to go. The MPPT must then shift the array operating point away from the peak power point until production exactly matches demand. An alternative approach for non MPPT systems and commonly used is to divert surplus PV power on a resistive load, allowing the array to operate continuously at its peak power-point. In this case, the system will follow the MPPT hysteresis lowering the value of the current draw from the PV modules in order to keep up with demand.


In a grid-tied photovoltaic system, the grid is essentially a battery with near-infinite capacity. The grid can always absorb surplus PV power, and it can cover shortfalls in PV production (e.g., at night). Batteries are needed only for protection from grid outages or in special applications to provide clean power. The MPPT is a grid-tied PV system that will always operate the array at its peak power point unless the grid fails when the batteries are full and there are insufficient local loads. It would then have to back the array away from its peak power point as in the off-grid case (which it has temporarily become). All this will be done following the MPPT hysteresis set of values specific to that configuration.

When a conventional controller is charging a discharged battery it simply connects the modules directly to the battery. This forces the modules to operate at battery voltage, typically not the ideal operating voltage at which the modules are able to produce their maximum available power. The PV Module Power/Voltage/Current graph shows the traditional Current/Voltage curve for a typical 75W module at standard test conditions of 25°C cell temperature and 1000W/m2 of isolation. This graph also shows the PV module power delivered vs module voltage.

For the example shown in the graph above, the conventional controller simply connects the module to the battery and therefore forces the module to operate at 12V. By forcing the 75W module to operate at 12V the conventional controller artificially limits power production to 53W. Rather than simply connecting the module to the battery, the MPPT system charge controller calculates the voltage at which the module is able to produce maximum power. In this example, the maximum power voltage of the module (VMP) is 17V. The MPPT system then operates the modules at 17V to extract the full 75W, regardless of actual battery voltage.

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