MPPT means maximum power point tracking. It is the job of finding the voltage and current at which a photovoltaic array delivers the most power at that moment, and of staying near that point as the light and the cell temperature change. The maximum power point is not a fixed voltage printed on the label. The label’s Vmp is the voltage at that point under standard test conditions. In the field the point moves. A tracker that cannot move with it leaves power on the table. A tracker is not, by itself, a charge controller, an inverter, or a promise of a percentage gain.
What the maximum power point is
A module’s current-voltage curve has a knee. Power, which is voltage times current, is highest at that knee. Left open, the module sits at open-circuit voltage and delivers no current, so no power. Shorted, it delivers current at almost no voltage, so again almost no power. Somewhere between, power peaks. Standard test conditions put a name on that peak: the nameplate wattage, at 1,000 watts per square meter and 25°C cell temperature, under the AM1.5 spectrum the efficiency tables use. Hot cells move the knee. Weak light moves it. The inverter or the controller has to follow.
Two different machines do the following. A solar inverter with MPPT runs the array at that point and converts the power to alternating current. A charge controller with MPPT runs the array at that point and converts the power to the battery’s voltage. A PWM charge controller does not. Victron describes PWM as a switch that pulls the array voltage down near the battery voltage. If the module’s power point is far above the battery, PWM never operates at the knee. In Victron’s example at 25°C, that cost 81 watts against 100 watts for the MPPT path, about 19 percent less for PWM in that example only. The gap shrinks in hot weather, because the module voltage has already fallen. The controller article, which is about the device rather than the point on the curve, is what a solar charge controller is.
What MPPT does not do
It does not recover a shaded string by magic. A series string has one current. A shaded module still limits that current unless the electronics are at the module, each with its own tracker. Several MPPT inputs on one inverter let you separate two orientations or two shade conditions. They do not put a tracker on every module unless you bought module-level hardware. Shade losses, and the 25 to 35 percent recovery of those losses for the module-level products NREL evaluated, are in how shade affects PV output. That recovery is a share of a loss. It is not what the letters MPPT mean.
It does not raise the module above its own curve. A tracker extracts the power the curve offers. Dirt, heat, and a low sun still lower the curve. People who see a hot-afternoon wattage below the nameplate and conclude the MPPT is broken are reading a temperature effect as a tracker fault. Heat is in how heat affects a solar module. A day that matches the clouds is weather: solar inverter faults and the weather.
It does not replace a voltage limit. The MPPT window is the range where the inverter will track. The maximum input voltage is a hard ceiling, often above the window, and the cold open-circuit voltage has to stay under that ceiling even before tracking starts. A string can be inside the window at noon and above the maximum at dawn. How strings add voltage is in what a solar string is.
How to read an MPPT line on a datasheet
| Line | What it is for |
|---|---|
| MPPT voltage range | Where the inverter will track. The string’s operating voltage, hot, should still sit inside it. A string that sags out of the window on a hot day produces poorly even though the modules are fine. |
| Maximum input voltage | The cold open-circuit limit. Not the same number as the top of the MPPT window. |
| Number of MPPT inputs | How many independent trackers you can assign to different orientations or shade. Unused inputs are not extra watts. |
| “MPPT efficiency 99 percent” | How well the tracker holds the knee, if the sheet defines it. Not the inverter’s conversion efficiency, and not a 99 percent gain over a shaded roof. |
Conversion efficiency is a different percentage: AC power out divided by DC power in, at a stated load. Peak conversion efficiency is one point on that curve. Do not add it to an MPPT efficiency and do not add either one to a module efficiency. They are not the same kind of number. Field efficiency, and why a datasheet percentage is not the day’s energy, is in what affects solar panel efficiency.
A single published controller comparison is easy to over-read. A 2019 test in Cúcuta on one 30-watt module found an MPPT controller with average efficiency 14.9 percent higher than the PWM controller in that test. Victron’s 19 percent gap was a different example. Neither number is the definition of MPPT. The definition is the knee of the curve, and the hardware that follows it.
Frequently asked questions
Is MPPT the same as an inverter?
No. MPPT is a function. Inverters and charge controllers can both include it. A battery inverter with no photovoltaic input does not track a module curve. A PWM controller charges a battery and does not track the knee.
Does more than one MPPT mean more energy on a clear roof?
Not if every module already sees the same sun and already sits on one tracker inside the voltage window. Extra inputs help when the roof has more than one orientation or more than one shade condition. They are not a multiplier.
Why does Vmp on the label differ from the voltage on the screen?
The label is the maximum power point at standard test conditions. The screen is the point the tracker has found at today’s irradiance and cell temperature. They should be related. They will not match at noon on a hot roof. If the screen is stuck at a limit, or one input is at open circuit while others work, then look for a fault.
Sources and references
Standard test conditions used for the records and the nameplate idea: Green and colleagues, efficiency tables version 68. PWM as a switch, and the 81-watt versus 100-watt example at 25°C: Victron. One 30-watt test in Cúcuta, MPPT average efficiency 14.9 percent higher in that test only: Colombia, 2019. Module-level shading recovery, which is not the definition of MPPT: NREL, 2016.
