Maximum Power Point Tracking (MPPT)

Solar charge controllers that come with maximum power point tracking (MPPT) are becoming widespread and less expensive. But many people don't know much about charge controllers in general, and even less about electronics and MPPT specifically. So we thought we would share our knowledge and experience on the subject.

Maximum power point tracking is not related to solar tracking.

Solar trackers move your array to point at the sun wherever it is in the sky, which can increase it's power output by a few percentage points, and help your system make power during dawn and dusk.

Maximum power point tracking has nothing to do with physical tracking, and has everything to do with the laws governing electricity. Specifically, Ohm's Law and Joule's Law.

The following may seem boring and/or confusing to some of you, but understanding these laws and the behavior of electricity is crucial to understanding how solar power works (and how it can be made more efficient).

Ohm's Law states that current, voltage, and resistance are measured proportionally. This law can be distilled into the following mathematical formula:

V = I x R

Where V = voltage, I = current, and R = resistance.

And knowing what we do about algebra, this formula can be rearranged to find a value for any one of the three elements. For example, to find current:

I = V / R

And to find resistance:

R = V / I

Joule's law states that power (P, which he measured as heat dissipated) is proportional to the current value squared and the resistance in the circuit. The law is distilled into the following equation:

P = I² x R

Thanks to Joule's Law and Ohm's Law, we know that power also has a special mathematical relationship to voltage and current.
 

P = I x V

Manipulating one value in the equation directly affects and changes the values of the others. And this very wordy explanation is leading to how MPPT charge controllers work, I promise.

So now that we know how the values relate to one another, we can manipulate the input value to optimize the power output.  This is usually illustrated with a PIV curve

So what a MPPT charge controller does is this:

The charge controller's electronics evaluate all the data it is receiving from the batteries and the solar panels, and adjusts the values for P, I, and V to the best possible benefit of your battery bank.

In doing so, the MPPT charge controllers increases your system's overall efficiency by a significant percentage.  Using an MPPT charge controller on our homestead's system has given us added power equivalent to an entire extra solar panel in the array. 

An added benefit of using an MPPT charge controller is that you do not have to match your array output voltage to your battery bank voltage.  Not only does it make the rules about sizing your system a whole lot less strict, this allows you to set up an array at high DC voltage and transmit the power farther, and on lighter gauge wire.

So you see, MPPT is a technology that opens up many doors to users of PV power.  It increases the efficiency of your array, netting you more useable power; it allows arrays and battery banks to be mismatched; and it allows us to use high voltage DC transmission lines from array to power center.